Quietly, frequently in foxy fashion (6)
I believe the answer is:
softly
'quietly' is the definition.
('softly' can be a synonym of 'quietly')
'frequently in foxy fashion' is the wordplay.
'frequently' becomes 'oft' (I've seen this before).
'in' means one lot of letters goes inside another.
'foxy fashion' becomes 'sly' (I can't explain this - if you can you should believe this answer much more).
'oft' inserted into 'sly' is 'SOFTLY'.
Can you help me to learn more?
(Other definitions for softly that I've seen before include "With a quiet and gentle voice" , "Lightly, quietly" , "In a yielding manner" , "Quietly or gently" , "In a quiet way" .)