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 am not sure about this - if you are sure you should give a lot more credence to this answer).
'pent' put into 'res' is 'REPENTS'.
'to be' acts as a 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" .)