Measure a horse (6)

I believe the answer is:

amount

'measure' is the definition.
(synonyms)

'a horse' is the wordplay.
'horse' becomes 'mount' (mount is a kind of horse).
'a'+'mount'='AMOUNT'

(Other definitions for amount that I've seen before include "Quantity, sum" , "Size" , "Measure, quantity" , "A quantity or total of anything" , "Total or extent" .)

I've seen this clue in The Sun, The Telegraph and The Times.

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