One working too hard pulls back (4)
I believe the answer is:
swot
'one working too hard' is the definition.
'pulls back' is the wordplay.
'pulls' becomes 'tows' (tow is a kind of pull).
'back' says the letters should be written backwards.
'tows' back-to-front is 'SWOT'.
(Other definitions for swot that I've seen before include "Person who studies hard" , "Learner" , "Student who studies too hard" , "Very keen pupil" , "Study intensively for an exam" .)