It may be used to describe days at school (4)
I believe the answer is:
term
'it may be used to describe days at school' is the definition.
The definition suggests a singular noun which matches the answer.
'it may be used' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'may be used' becomes 'erm' (I can't explain this - if you can you should give a lot more credence to this answer).
't'+'erm'='TERM'
Can you help me to learn more?
(Other definitions for term that I've seen before include "Period of education" , "Word; trimester" , "Word or expression and period of time" , "Time spent at school" , "School time; word" .)