Thoughtless prisoner lies about pinching book (13)
I believe the answer is:
Here is my best explanation:
'thoughtless' is the definition.
('irresponsible' can be similar in meaning to 'thoughtless')
'prisoner lies about pinching book' is the subsidiary indication.
'about' indicates anagramming the letters.
'pinching' is an insertion indicator (taking inside).
'book' becomes 'b'.
'prisonerlies' anagrammed gives 'irresponsile'.
'irresponsile' enclosing 'b' is 'irresponsible'.
(Other definitions for irresponsible that I've seen before include "Careless", "Reprehensibly careless", "Showing a lack of care for consequences", "Negligent", "Nor caring about consequences", "Unreliable", "Thoughtless" and "Wantonly careless".)