Continue being eager to hold work back (4,2)
I believe the answer is:
keep on
'continue' is the definition.
(I know that continue is a more specific form of the action keep up)
'eager to hold work back' is the wordplay.
'eager' becomes 'keen' (I've seen this in another clue).
'to hold' means one lot of letters goes inside another.
'work' becomes 'op' (operation, work done in an operating theatre).
'back' shows that the letters should be reversed in order.
'op' back-to-front is 'po'.
'keen' enclosing 'po' is 'KEEP ON'.
'being' acts as a link.
(Other definitions for keep on that I've seen before include "Continue; retain in service" , "Continue, persevere" , "Continue; not dismiss" .)