Only one pass for a number of soldiers (6)

I believe the answer is:

defile

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

'only one pass' is the definition.
'defile' can be an answer for 'pass' (defile is a kind of pass). I'm unsure of the 'only one' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for defile that I've seen before include "Befoul" , "passage" , "dishonour?" , "Soil" , "Mar; narrow gorge" .)

I've seen this clue in the Evening Standard.

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