Certain the crook was friendly on the outside, as an aside (15)
I believe the answer is:
parenthetically
'as an aside' is the definition.
(I've seen this before)
'certain the crook was friendly on the outside' is the wordplay.
I cannot quite see how this works, but
'certain' could be 'the' (definite article - refers to a certain object) and 'the' is found within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for parenthetically that I've seen before include "(sic)" , "as an aside" , "A number here may appear this way!" .)