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' is an anagram indicator.
'denis' with letters rearranged 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" .)