hammer away at (?.)
- always trot out
- apply the mind
- belabor
- dig
- drudge
- dwell on
- dwell upon
- fag
- grind
- grub
- hammer
- hammer at
- hammer away
- harp on
- insist upon
- keep doggedly at
- labor
- moil
- peg
- peg away
- plod
- plug
- plug along
- plug at
- plug away
- pound away
- puzzle
- puzzle over
- slog
- slog away
- stew over
- think hard
- thresh over
- toil
- travail
- wade through
- work away