Possible Clues:

• Key to fire-lighting
• System for firing one caught in observing another one from behind
• Mechanism that fires the fuel in an internal-combustion engine
• Combustion's start
• Starter key
• Some sign it ionised, creating spark
• One getting fired for scrubbing Alps from oil paintings
• Fire engine must need this
• Firing international, into gin unfortunately
• Mechanism firing up an internal-combustion engine