Related Clues:

• Butting heads
• Engaged
• Going ___ tooth and nail
• Keep ___ (persevere)
• On the job
• Working diligently
• Wrangling
• Arguing loudly
• Feuding
• Persisting
• Having one's nose to the grindstone
• Keep -- (persist)
• Locking horns
• Fighting, often with 'again'
• Up to no good