Group with a squad kept in reserve (3,5)
I believe the answer is:
set aside
'reserve' is the definition.
(synonyms)
'group with a squad' is the wordplay.
'group' becomes 'set' (both can mean a collection).
'with' means one lot of letters go next to another.
'squad' becomes 'side'.
'set'+'a'+'side'='SET-ASIDE'
'kept in' acts as a link.
(Other definitions for set aside that I've seen before include "Preserve" , "Policy for leaving land alone" , "not productive land" , "Reserve(d) in advance" , "Lay by" .)