Supported team, on the day (5)
I believe the answer is:
sided
'supported' is the definition.
(I've seen this in another clue)
'team on the day' is the wordplay.
'team' becomes 'side' (both can mean a sports team).
'on' means one lot of letters go next to another.
'the day' becomes 'd' (abbreviation for day).
'side'+'d'='SIDED'
(Other definitions for sided that I've seen before include "Showed solidarity (with)" , "Allied oneself (with one party)" , "Took part with person in a dispute" .)