Crossword Genius

What the streaking soldier wore? (7)

Ross

I believe the answer is:

undress

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

'what the streaking' is the definition.
I can't tell whether this definition defines the answer.

'soldier wore?' is the wordplay.
I cannot really understand how this works, but
'soldier' could be 're' (Royal Engineer) and 're' is found within the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for undress that I've seen before include "Disrobe - uniform for ordinary occasions" , "Strip, take off clothes" , "Remove one's clothes" , "Take one's clothes off" , "Take off clothing, strip" .)

I've seen this clue in the Evening Standard.
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