Sick English prisoner: revolting criminal (7)
I believe the answer is:
'criminal' is the definition.
('illegal' can be similar in meaning to 'criminal')
'sick english prisoner revolting' is the wordplay.
'sick' becomes 'ill' ('ill' can be a synonym of 'sick').
'english' becomes 'e' (abbreviation).
'prisoner' becomes 'lag' (I've seen this before).
'revolting' says the letters should be written in reverse.
'lag' written backwards gives 'gal'.
(Other definitions for illegal that I've seen before include "Against the rules" , "Black-market" , "Not lawful" , "Unlawful" , "crooked" .)