Continue working after brief look back (4,2)
I believe the answer is:
keep on
'continue' is the definition.
(synonyms)
'working after brief look back' is the wordplay.
'working' becomes 'on' (e.g. a machine is working or on).
'after' is a charade indicator (letters next to each other).
'brief look' becomes 'peek' (I've seen this before).
'back' shows that the letters should be reversed in order.
'peek' in reverse letter order is 'keep'.
'on' put after 'keep' is 'KEEP ON'.
(Other definitions for keep on that I've seen before include "Continue; not dismiss" , "Continue; retain in service" , "Continue, persevere" .)