Study briefly (5)
I believe the answer is:
maths
'study' is the definition.
(maths is a kind of study)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for maths that I've seen before include "18 across subject" , "The science of number, quantity and space" , "subject, in short" , "Numbers subject" , "Study of numbers and sums" .)