"A" team in reserve (5)
I believe the answer is:
'in reserve' is the definition.
(I've seen this before)
'a team' is the wordplay.
'team' becomes 'side' (I've seen this before).
(Other definitions for aside that I've seen before include "Actors comment to his audience" , "Apart; incidental remark" , "Privately, apart" , "Actor's line directed solely to audience" , "Whispered comment" .)