I met the crooked Denis in prison (6)
I believe the answer is:
inside
'in prison' is the definition.
(I've seen this before)
'i met the crooked denis' is the wordplay.
'met' means one lot of letters go next to another.
'the crooked' indicates anagramming the letters (a crooked or disrupted version of the letters).
'denis' anagrammed gives 'nside'.
'i'+'nside'='INSIDE'
(Other definitions for inside that I've seen before include "Imprisoned" , "serving porridge" , "close to centre" , "Inner surface" , "banged up" .)