Related Clues:

• Deep in thought
• Persisting
• Having one's nose to the grindstone
• Totally absorbed
• Keep -- (persist)
• En-grossed
• Locking horns
• Fighting, often with 'again'
• Up to no good
• Keep ___ (persist)
• Go ___ (tussle)
• In a row
• Like a fan during overtime
• Unblinking
• Keep __ (persist)