Horse, beginning slowly, canters and gallops (4)

I believe the answer is:

scag

'horse' is the definition.
('scag' can be a synonym of 'horse')

'beginning slowly canters and gallops' is the wordplay.
'beginning' says to take the initial letters.
The initial letters of 'slowly canters and gallops' is 'SCAG'.

(Other definitions for scag that I've seen before include "heroin" , "Sugar" .)

I've seen this clue in The Times.

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