A fop to boot (2,4)
I believe the answer is:
as well
'to boot' is the definition.
(I've seen this before)
'a fop' is the wordplay.
'fop' becomes 'swell' (synonyms).
'a'+'swell'='AS WELL'
(Other definitions for as well that I've seen before include "Also, additionally" , "To boot" , "Also, in addition" , "with equal effect" , "on top" .)