Is sorry to be confined in the reserves (7)
I believe the answer is:
'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' means one lot of letters goes inside another.
'the reserves' becomes 'res' (I can't explain this - if you can you should give a lot more credence to this answer).
'pent' inserted within '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 "Turns away from evil", "Feels remorse for, sin perhaps", "Present (anag.)", "Expresses sorrow for sin", "Feels sincere remorse".)