Remains unwanted at first in army corps team (7)
I believe the answer is:
residue
'remains' is the definition.
(I've seen this before)
'unwanted at first in army corps team' is the wordplay.
'at first' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'army corps' becomes 're' (I've seen this in other clues).
'team' becomes 'side' (both can mean a sports team).
The initial letter of 'unwanted' is 'u'.
're'+'side'='reside'
'u' going within 'reside' is 'RESIDUE'.
(Other definitions for residue that I've seen before include "waste" , "What is left, maybe dregs" , "Deposit" , "Dire use of the remainder" , "Material left" .)