To begin with, pupils rarely enjoy physics homework (4)
I believe the answer is:
prep
'homework' is the definition.
('prep' can be a synonym of 'homework')
'to begin with pupils rarely enjoy physics' is the wordplay.
'to begin with' says to take the initial letters.
The first letters of 'pupils rarely enjoy physics' is 'PREP'.
(Other definitions for prep that I've seen before include "School work done outside lessons" , "warm-up race" , "Homework (abbr.)" , "Homework (abbrev.)" .)