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' shows that the letters should be reversed in order.
'in' indicates the answer is hidden within the clue.
'egral' is hidden within 'integral calculus'.
'egral' written backwards gives 'LARGE'.
(Other definitions for large that I've seen before include "Huge" , "Generous" , "broad" , "Gross" , "substantial" .)