Look back, working hard, and persevere (4,2,2)
I believe the answer is:
keep at it
'persevere' is the definition.
(to keep at something is to persevere with it)
'look back working hard' is the wordplay.
'look' becomes 'peek' (peek is a kind of look).
'back' says the letters should be written backwards.
'working hard' becomes 'at it' (someone working on a particular task is 'at it').
'peek' back-to-front is 'keep'.
'keep'+'atit'='KEEP AT IT'
'and' is the link.
(Other definitions for keep at it that I've seen before include "Continue" , "Persevere" , "carry on" .)