Is sorry to be confined in the reserves (7)
I believe the answer is:
repents
'is sorry' is the definition.
(I've seen this in another clue)
'confined in the reserves' is the wordplay.
'confined' becomes 'pent' (similar in meaning).
'in' is an insertion indicator.
'the reserves' becomes 'res' (I can't justify this - if you can you should believe this answer much more).
'pent' inserted within 'res' is 'REPENTS'.
'to be' is the link.
Can you help me to learn more?
(Other definitions for repents that I've seen before include "Feels sincere remorse" , "Present (anag.)" , "Expresses sorrow for sin" , "Turns away from evil" , "Feels remorse for, sin perhaps" .)