Prison officer bragged after start of sentence (5)
I believe the answer is:
screw
'prison officer' is the definition.
(slang term)
'bragged after start of sentence' is the wordplay.
'bragged' becomes 'crew' (past tense of 'crow').
'after' means one lot of letters go next to another.
'start of' suggests taking the first letters.
The initial letter of 'sentence' is 's'.
'crew' put after 's' is 'SCREW'.
(Other definitions for screw that I've seen before include "Take advantage of" , "Metal fastener" , "Helical thread" , "shaft" , "something with thread" .)