Be sorry about being shut up (6)
I believe the answer is:
repent
'be sorry' is the definition.
(I've seen this before)
'about being shut up' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'being shut up' becomes 'pent' ('pent' can be a synonym of 'shut up').
're'+'pent'='REPENT'
(Other definitions for repent that I've seen before include "Feel or express contrition" , "Feel sincere regret" , "Be sorry for wrongdoing" , "Turn away from sin" , "John the Baptist's cry" .)