A leading part as an informer (5)

I believe the answer is:

snout

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'a leading part' is the definition.
'snout' can be an answer for 'part' (snout is a kind of part). I'm not sure about the 'a leading' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for snout that I've seen before include "Muzzle; Wall (MND)" , "Pig's nose" , "Projecting nasal area of animal" , "Tobacco -- hooter" , "Animal's long, projecting nose" .)

I've seen this clue in the Evening Standard.

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