Great he may be, but he gets some quantities wrong (5)
I believe the answer is:
titan
'great' is the definition.
(I've seen this before)
'he may be but he gets some quantities wrong' is the wordplay.
I cannot really understand how this works, but
'he' could be 'tan' and 'tan' is located in the answer.
The remaining letters 'ti' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for titan that I've seen before include "colossal one" , "Atlas, possibly" , "Saturn's largest satellite" , "Huge(ly important) type" , "One of elder gods defeated by Zeus" .)