Besides being a toff (2,4)
I believe the answer is:
as well
'besides' is the definition.
(thesaurus)
'a toff' is the wordplay.
'toff' becomes 'swell' (wealthy or high-class person).
'a'+'swell'='AS WELL'
'being' acts as a link.
(Other definitions for as well that I've seen before include "Also, in addition" , "with equal effect" , "on top" , "Appropriate" , "Also, additionally" .)