Fools having evil thoughts (7)
I believe the answer is:
musings
'thoughts' is the definition.
(musing is a kind of thought)
'fools having evil' is the wordplay.
'fools' becomes 'mugs' ('mug' can be a synonym of 'fool').
'having' means one lot of letters goes inside another.
'evil' becomes 'sin' (synonyms).
'mugs' placed around 'sin' is 'MUSINGS'.
(Other definitions for musings that I've seen before include "dreams" , "Thoughts" .)