Suit completely but not regularly (8)
I believe the answer is:
fitfully
'not regularly' is the definition.
Both the definition and answer are adverbs. Perhaps there's an association between them I don't understand?
'suit completely' is the wordplay.
'suit' becomes 'fit' (synonyms).
'completely' becomes 'fully' (in full).
'fit'+'fully'='FITFULLY'
'but' is the link.
Can you help me to learn more?
(Other definitions for fitfully that I've seen before include "not consistently" , "Spasmodically" , "occurring in bouts?" , "Only at times" , "Intermittently" .)