One heartily slapped? (5)

I believe the answer is:

thigh

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'one heartily slapped?' is the definition.
I can't judge whether this definition defines the answer.

'one heartily slapped?' is the wordplay.
'one' becomes 'unitary' (synonyms).
'heartily' means to look at the middle letters.
'slapped?' becomes 'high' (I am not sure about this - if you are sure you should believe this answer much more).
The middle of 'unitary' is 't'.
't'+'high'='THIGH'

Can you help me to learn more?

(Other definitions for thigh that I've seen before include "Ham" , "Joint of meat" , "top of pin" , "Part of member" , "Upper joint of the leg" .)

I've seen this clue in the Evening Standard.

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