Getting lined up for first aid (8)

I believe the answer is:

dressing

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'aid' is the definition.
(I know that dressing is a type of treatment)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for dressing that I've seen before include "Saucy thing" , "but not stripping!" , "bedroom procedure?" , "plaster, for example?" , "Sauce for salads" .)

I've seen this clue in the Evening Standard.

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