Related Clues:

• Begin to use file, for example, starting off prison escape nicely
• Lead-in to "sesame" or "secret"
• Inviting store sign
• Like an unenclosed floor plan
• Word on a shop sign
• Unlocked, as a door
• Word before "door" or "house"
• Sign on a shop window
• Unseal, as an envelope
• Like an inviting door
• Major competition seeing Hearts in proper arenas
• 'Yes, we're ___' (store sign)
• Like some non-monogamous relationships
• Frank not to start competition anyone can enter
• Mining method