Clavarino was drafted by St Kilda with pick 35 in the 2017 national draft. Clavarino was St Kilda's third selection; the club had already taken
Hunter Clark and
Nick Coffield with picks 7 and 8 and went on to choose
Ben Paton with pick 46. St Kilda sent Clavarino's new teammate
David Armitage around to his home to present him with a club
guernsey. Clavarino spent his early seasons with St Kilda developing his strength to AFL standards. Describing himself as a "tall and skinny" player in his reflections, Clavarino did not expect to play at AFL level for a few seasons. He named several mentors at St Kilda: among the staff,
Aaron Hamill, the VFL coach; and Steve Forcone, the strength and conditioning coach. Among the players, he named defensive veterans
Nathan Brown and
Dylan Roberton. In February 2019, Clavarino injured his ankle in a marking contest during an intra-club practice match. The injury sidelined him for eight weeks. This was a serious setback – Clavarino had been anticipated to make his AFL debut in the opening round of the season, since Brown was suspended and fellow key defender
Jake Carlisle was recovering from an injured back. Clavarino ended up playing 14 VFL matches with Sandringham for the year, but did not play an AFL game. He signed a one-year contract extension in October;
Simon Lethlean, the club's general manager of football, acknowledged Clavarino's position as a "bigger-bodied defender" meant he would take some time to develop as a player. Clavarino's development was further interrupted in 2020 by the emerging
COVID-19 pandemic.
The AFL season was suspended for more than two months and only resumed under strict quarantine arrangements –
the VFL season was cancelled altogether. In the absence of reserves football, Clavarino pressed his case for AFL selection through the practice games arranged against other clubs. These
scratch matches were played under relaxed rules and reduced numbers; on one occasion, Clavarino and some teammates played for
Essendon to balance their match. The pandemic was not the only interruption to Clavarino's progress; he also suffered a hamstring injury in June, which kept him out of football for several weeks. In May 2021, Clavarino finally played his first AFL match, 1283 days after being drafted; a twenty-point victory over
North Melbourne in round 11. Forcone delivered him the news of his selection at a team meeting.
Victoria was still in COVID-19 lockdown and AFL matches in the state were played without crowds, so Clavarino's family was unable to attend the game; St Kilda set up a videoconference so they could watch his guernsey presentation. Clavarino played in St Kilda's final two matches for the year, ending with a 58-point win over
Fremantle. During his time at the club, he played five AFL matches and 39 VFL matches. ==State-league football==