Prison rebel is terribly reckless (13)
I believe the answer is:
irresponsible
'reckless' is the definition.
('irresponsible' can be a synonym of 'reckless')
'prison rebel is terribly' is the wordplay.
'terribly' is an anagram indicator.
'prison'+'rebel'+'is'='prisonrebelis'
'prisonrebelis' anagrammed gives 'IRRESPONSIBLE'.
(Other definitions for irresponsible that I've seen before include "Unreliable" , "Negligent" , "Nor caring about consequences" , "Showing a lack of care for consequences" , "Wantonly careless" .)