There's nothing in returning a kiss (3)

I believe the answer is:

nix

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

'there's nothing' is the definition.
I can't judge whether this defines the answer.

'in returning a kiss' is the wordplay.
'returning' is a reversal indicator.
'a kiss' becomes 'x' (X can represent a kiss).
'in' written backwards gives 'ni'.
'ni'+'x'='NIX'

Can you help me to learn more?

(Other definitions for nix that I've seen before include "Nothing (sl.)" , "Spiteful water spirit" , "Watch out for" , "water sprite" , "0" .)

I've seen this clue in the Evening Standard.

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