As a gesture of disapproval, left muggins out! (4)

I believe the answer is:

slap

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'as a gesture of disapproval' is the definition.
I can't judge whether this defines the answer.

'left muggins out' is the wordplay.
'left' becomes 'l' (common abbreviation).
'muggins' becomes 'sap' (synonyms).
'out' indicates putting letters inside.
'l' inserted into 'sap' is 'SLAP'.

Can you help me to learn more?

(Other definitions for slap that I've seen before include "Right" , "Mild punishment" , "A blow among pals" , "Strike (in the face)" , "Strike with the palm" .)

I've seen this clue in the Evening Standard.

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