A dandy to boot (2,4)
I believe the answer is:
as well
'to boot' is the definition.
(I've seen this before)
'a dandy' is the wordplay.
'dandy' becomes 'swell' (both can informally mean excellent or satisfactory).
'a'+'swell'='AS WELL'
(Other definitions for as well that I've seen before include "Appropriate" , "A swell sort of addition" , "Also, in addition" , "on top" , "To boot" .)