Chose team on the day (5)
I believe the answer is:
sided
'chose' is the definition.
(siding is a kind of choosing)
'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 (in a down clue, letters appear on others).
'the day' becomes 'd' (abbreviation for day).
'side'+'d'='SIDED'
(Other definitions for sided that I've seen before include "Allied oneself (with one party)" , "Took part with person in a dispute" , "Showed solidarity (with)" .)