First grade team in reserve (5)
I believe the answer is:
aside
'in reserve' is the definition.
(synonyms)
'first grade team' is the wordplay.
'first' becomes 'a' (first letter of the alphabet).
'grade team' becomes 'side' (both can mean a sports team. I am not sure about the 'grade' bit.).
'a'+'side'='ASIDE'
Can you help me to learn more?
(Other definitions for aside that I've seen before include "(Pushed) out of the way" , "In reserve; actor's remark to audience" , "Stage whisper" , "secretly" , "Out of other people's hearing" .)