Prepare a team reserve (3,5)
I believe the answer is:
set aside
'reserve' is the definition.
('set aside' can be a synonym of 'reserve')
'prepare a team' is the wordplay.
'prepare' becomes 'set' ('set' can be a synonym of 'prepare'**).
'team' becomes 'side' (I've seen this before).
'set'+'a'+'side'='SET-ASIDE'
(Other definitions for set aside that I've seen before include "Preserve" , "The policy of taking land out of production to reduce crop surplus" , "Discount" , "not productive land" , "The policy of taking land out of production to reduce crop surpluses" .)