Surplus is carried over (2,4)
I believe the answer is:
de trop
'surplus' is the definition.
(thesaurus)
'carried over' is the wordplay.
'carried' becomes 'ported' (porting is a kind of carrying).
'over' shows that the letters should be reversed in order.
'ported' backwards is 'DE TROP'.
'is' is the link.
(Other definitions for de trop that I've seen before include "Superfluous (French)" , "Deport from France, being in the way?" , "Unwanted - excessive (French)" , "Excessive - not wanted" , "Unnecessary" .)