He left the oil to work (4)
I believe the answer is:
toil
'work' is the definition.
(toil is a kind of work)
'he left the oil' is the wordplay.
'left' is a deletion indicator.
'the' with 'he' taken away is 't'.
't'+'oil'='TOIL'
'to' is the link.
(Other definitions for toil that I've seen before include "Work incessantly" , "Work hard, labour" , "Work extremely hard" , "Work hard and long" , "Hard physical work" .)