Slap person tailing gumshoe? (9)
I believe the answer is:
eyeshadow
'slap' is the definition.
(slap can mean make-up)
'person tailing gumshoe?' is the wordplay.
I cannot really see how this works, but
'person' could be 'shadow' (shadow is a kind of person) and 'shadow' is present in the answer.
The remaining letters 'eye' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for eyeshadow that I've seen before include "Kind of warpaint?" , "used to put dark side into orbit" , "application to viewers" , "Face application" , "part of make-up" .)