Keep one plugging away? Hardly (6)
I believe the answer is:
adhere
'keep' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
(Other definitions for adhere that I've seen before include "Stick or cleave to" , "Stick like a stamp" , "Stick to or continue faithful" , "Cling, attach" , "Agree" .)