One caught it sitting by poorly criminal (7)
I believe the answer is:
illicit
'criminal' is the definition.
(I've seen this before)
'one caught it sitting by poorly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'caught' becomes 'c' (cricket abbreviation).
'sitting by' means one lot of letters go next to another.
'poorly' becomes 'ill' (synonyms).
'i'+'c'+'it'='icit'
'icit' after 'ill' is 'ILLICIT'.
(Other definitions for illicit that I've seen before include "Not lawful" , "Felonious" , "Forbidden by law, rules" , "Not allowed, unlawful" , "Contrary to morality or convention" .)