*Spending limit (4)
(Other definitions for carp that I've seen before include "Fish; nag pettily" , "Pond fish" , "Fish - grouse" , "Nagging fish?" , "Cavil - fish" .)
'spending limit' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cap that I've seen before include "contribution to hunt funds?" , "Soft headgear" , "Top; hat" , "'If the . . . fits, wear it' (3" , "Flat brimless hat" .)