Spot Royal Marines involved in dress-up (5)

I believe the answer is:

primp

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'spot royal marines involved' is the definition.
I can't tell whether this definition defines the answer.

'dress-up' is the wordplay.
'dress' becomes 'prim' (primming is a kind of dressing).
'up' becomes 'p' (I can't justify this - if you can you should give a lot more credence to this answer).
'prim'+'p'='PRIMP'

'in' acts as a link.

Can you help me to learn more?

(Other definitions for primp that I've seen before include "Make minor adjustments to one's appearance" , "Groom oneself elaborately" , "Make small adjustments to one's appearance" , "Smarten up" , "Make minor changes to one's appearance" .)

I've seen this clue in the Sydney Morning Herald.

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