I left to start work (4)
I believe the answer is:
toil
'work' is the definition.
(I know that toil is a more specific form of the action work)
'i left to start' is the wordplay.
'left' becomes 'l' (common abbreviation).
'to' means one lot of letters go next to another (I've seen this in other clues).
'start' becomes 'to' (I can't explain this - if you can you should believe this answer much more).
'i'+'l'='il'
'il' put after 'to' is 'TOIL'.
Can you help me to learn more?
(Other definitions for toil that I've seen before include "Work hard, labour" , "Work really hard" , "Drudgery" , "Work extremely hard" , "Work incessantly" .)