Possible Clues:

• During Lent is the time to be in force
• That's the way to listen, if you want to join forces
• Has the hen lost her head and be inclined to join forces
• One might have sent about fifty-one like this to join up
• Collect, as volunteers
• Listen, that's one way to join forces
• What the silent joiner may do
• Recruit's silent break
• 16 (anag.)
• If you wish to join forces, listen like this