Crossword Genius

Persist if flying planes in war (9)

Ross

I believe the answer is:

spitfires

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'planes in war' is the definition.
Although both the answer and definition are plural nouns, I don't understand how one could define the other.

'persist if flying' is the wordplay.
'flying' is an anagram indicator (letters fly around into new positions).
'persist'+'if'='persistif'
'persistif' with letters rearranged gives 'SPITFIRES'.

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I've seen this clue in The Guardian.
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