This pony wasted time - he'll put you under (9)

I believe the answer is:

hypnotist

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'he'll put you under' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

'this pony wasted time' is the wordplay.
'wasted' indicates an anagram.
'time' becomes 't' (abbreviation).
'this'+'pony'='thispony'
'thispony' anagrammed gives 'hypnotis'.
'hypnotis'+'t'='HYPNOTIST'

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(Other definitions for hypnotist that I've seen before include "charmer" , "Mesmeriser" , "One who mesmerises" , "stage entertainer" , "Mesmerising person" .)

I've seen this clue in The Daily Express.

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