Not brilliant at figures (3)
I believe the answer is:
dim
'not brilliant' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for dim that I've seen before include "Opaque, not bright" , "Not bright, cloudy" , "thick" , "Unpromising" , "Indistinct, dark" .)