Some prisoner up to break out (5)
I believe the answer is:
erupt
'break out' is the definition.
(synonyms)
'some prisoner up to' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'ERUPT' can be found hidden inside 'prisoner up to'.
(Other definitions for erupt that I've seen before include "Burst forth violently" , "What canines do" , "Break through (as a volcano)" , "Break forth" , "Blow up like volcano" .)