Pondering evil, sucker goes outside (6)
I believe the answer is:
musing
'pondering' is the definition.
('muse' can be a synonym of 'ponder')
'evil sucker goes outside' is the wordplay.
'evil' becomes 'sin' (I've seen this before).
'sucker' becomes 'mug' ('mug' can be a synonym of 'sucker'**).
'goes outside' is an insertion indicator.
'sin' inserted into 'mug' is 'MUSING'.
(Other definitions for musing that I've seen before include "Reflecting or pondering" , "Meditating" , "introspection" , "reflection" , "Absorbed in thought" .)