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