Some schoolmaster missing for part of school year (4)
I believe the answer is:
term
'part of school year' is the definition.
(I've seen this before)
'some schoolmaster missing for' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'TERM' is hidden in the letters of 'schoolmaster missing for'.
(Other definitions for term that I've seen before include "Time spent at school" , "Word; trimester" , "Call" , "Limited period of time" , "Word or expression and period of time" .)