Get behind team that's bottom (8)
I believe the answer is:
backside
'that's bottom' is the definition.
(synonyms)
'get behind team' is the wordplay.
'get' is a charade indicator (letters next to each other).
'behind' becomes 'back' (similar in meaning).
'team' becomes 'side' (both can mean a sports team).
'back' put next to 'side' is 'BACKSIDE'.
(Other definitions for backside that I've seen before include "Rear" , "Behind" , "Buttocks" , "Seat" , "Bottom" .)