Homework before start of playtime (4)
I believe the answer is:
prep
'homework' is the definition.
('prep' can be a synonym of 'homework')
'before start of playtime' is the wordplay.
'before' becomes 'pre'.
'start of' says to take the initial letters.
The first letter of 'playtime' is 'p'.
'pre'+'p'='PREP'
(Other definitions for prep that I've seen before include "Homework (abbr.)" , "School work done outside lessons" , "warm-up race" , "Homework (abbrev.)" .)