St Clair joined
Chelsea as an 8-year-old, and played for their under-18 team when he was 15. After injury ruled him out of the opening weeks of the season, St Clair made his
Serie B debut on 30 October as an 88th-minute substitute for
Francesco Di Mariano in a 1–0 away win over
Cremonese. He made his first Serie B start on 30 March 2019 in a 1–1 draw away to
Salernitana. On 2 September 2019, St Clair signed for
Scottish Premiership club
Kilmarnock on a season-long loan. He made his debut twenty days later in a 3–1 loss at holders
Celtic, as a 72nd-minute substitute for
Liam Millar. On 31 January 2022, St Clair joined
Serie C Triestina on loan. On 27 January 2023, St Clair moved to
Vis Pesaro in
Serie C and signed a contract until the end of the season. On 1 September 2023, he left Venezia by mutual consent. On 7 February 2024, St Clair signed with
FC Tulsa for the
2024 USL Championship season. == International career ==