Reserve is assigned a team (3,5)
I believe the answer is:
set aside
'reserve' is the definition.
(I know that reserve can be written as set aside)
'assigned a team' is the wordplay.
'assigned' becomes 'set' (I've seen this in another clue).
'team' becomes 'side' (both can mean a sports team).
'set'+'a'+'side'='SET-ASIDE'
'is' is the link.
(Other definitions for set aside that I've seen before include "kept for later" , "not productive land" , "Farm land taken out of production" , "Save or discard" , "Policy for leaving land alone" .)