Big setback in integral calculus (5)
I believe the answer is:
large
'big' is the definition.
(I know that big can be written as large)
'setback in integral calculus' is the wordplay.
'setback' says the letters should be written backwards.
'in' indicates the answer is hidden within the clue.
'egral' can be found hidden inside 'integral calculus'.
'egral' reversed gives 'LARGE'.
(Other definitions for large that I've seen before include "Huge" , "Generous" , "broad" , "Gross" , "substantial" .)