Crossword Genius

Fire on plane when phosphorus leaks from unopened backpack (3-3)

Ross

I believe the answer is:

ack-ack

I'm a little stuck... Click here to teach me more about this clue!

'fire on plane' is the definition.
'ack ack' can be an answer for 'fire' (I've seen this before). I'm unsure of the 'on plane' bit.

'phosphorus leaks from unopened backpack' is the wordplay.
'phosphorus leaks' becomes 'ack' (I can't justify this - if you can you should give a lot more credence to this answer).
'from' says to put letters next to each other.
'unopened' means to remove the first letter (without opening letter).
'backpack' becomes 'pack' (synonyms).
'pack' with its first letter taken off is 'ack'.
'ack'+'ack'='ACK-ACK'

'when' is the link.

Can you help me to learn more?

(Other definitions for ack-ack that I've seen before include "Anti-aircraft gun" , "Anti-aircraft gunfire" , "Anti-aircraft fire" .)

I've seen this clue in The Independent.
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