Reject ready to join first-rate team (3,5)
I believe the answer is:
set aside
'reject' is the definition.
(thesaurus)
'ready to join first-rate team' is the wordplay.
'ready to' becomes 'set' (synonyms).
'join' means one lot of letters go next to another.
'first-rate' becomes 'A'.
'team' becomes 'side' (both can mean a sports team).
'set'+'a'+'side'='SET-ASIDE'
(Other definitions for set aside that I've seen before include "The policy of taking land out of production to reduce crop surpluses" , "kept for later" , "Reserve(d) in advance" , "Reserve for future use" , "Policy for leaving land alone" .)