Small lions occasionally misbehave (3)
I believe the answer is:
sin
'misbehave' is the definition.
(I've seen this in another clue)
'small lions occasionally' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'occasionally' indicates alternate letters.
The alternating letters of 'lions' are 'in'.
's'+'in'='SIN'
(Other definitions for sin that I've seen before include "Immoral action" , "Immorality" , "Wicked deed" , "Moral offence" , "Wickedness" .)