Subject taught by a master for some months (5)
I believe the answer is:
maths
'subject' is the definition.
(maths is a kind of subject)
'taught by a master for some months' is the wordplay.
I cannot quite understand how this works, but
'by' could be 'at' and 'at' is present in the answer.
'master' could be 'm' and 'm' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for maths that I've seen before include "Arithmetic etc" , "Study of numbers and sums" , "The science of number, quantity and space" , "Science of numbers etc" , "A school subject (abbr.)" .)