Contemplating a fool about to transgress (6)
I believe the answer is:
musing
'contemplating' is the definition.
('muse' can be a synonym of 'contemplate')
'a fool about to transgress' is the wordplay.
'a fool' becomes 'mug' (synonyms).
'about' is an insertion indicator.
'to transgress' becomes 'sin' (sinning is a kind of transgressing).
'mug' going around 'sin' is 'MUSING'.
(Other definitions for musing that I've seen before include "Cogitation" , "Reflecting in silence" , "introspection" , "Meditating" , "Reflecting or pondering" .)