Mathematical computation, hard stuff (8)
I believe the answer is:
calculus
'mathematical computation' is the definition.
'calculus' can be an answer for 'computation' (thesaurus). I'm not sure about the 'mathematical' bit.
'hard stuff' is the wordplay.
I cannot quite understand how this works, but
'stuff' could be 'cu' (Cu is an example) and 'cu' is located in the answer.
The remaining letters 'callus' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for calculus that I've seen before include "Branch of maths involving rates of variation" , "method of reasoning" , "Branch of maths dealing with differentiation and integration" , "Maths with derivatives" , "Branch of mathematics developed by Newton and Leibnitz" .)