Mental ability to understand, say, a multi-dimensional surface (5)
I believe the answer is:
brain
'mental ability' is the definition.
'brain' can be an answer for 'ability' (brain is a kind of ability). I'm unsure of the 'mental' bit.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for brain that I've seen before include "Intellectual ability" , "Clever person" , "The grey matter" , "It's in the skull" , "and 15 ac. The loss of experts through emigration" .)