A couple of fifties provides the sum (3)
I believe the answer is:
all
'the sum' is the definition.
(thesaurus)
'a couple of fifties' is the wordplay.
'couple of fifties' becomes 'll' (two L's, Roman numerals for fifty).
'a'+'ll'='ALL'
'provides' is the link.
(Other definitions for all that I've seen before include "'Bless 'em - - ' (song)" , "Garden passage between trees or bushes" , "nothing more" , "Long avenue or garden path" , "that's as many as there are" .)