It has its limits (4)
(Other definitions for calculus that I've seen before include "Branch of maths involving rates of variation" , "Maths with derivatives" , "Branch of maths dealing with differentiation and integration" , "method of reasoning" , "Branch of mathematics developed by Newton and Leibnitz" .)
'it has its limits' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for city that I've seen before include "Town with cathedral" , "Big conurbation" , "eg New York" , "Big town" , "See 2" .)