What sides in Crécy might indicate? Conflict, a long one (7,5,3)

I believe the answer is:

hundred years war

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'conflict a long one' is the definition.
'hundred years war' can be an answer for 'conflict' (I've seen this before). I'm not sure about the 'a long one' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for hundred years war that I've seen before include "historic conflict" , "drawn-out conflict" .)

I've seen this clue in The Times.

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