Gentle, as a horse (4)
I believe the answer is:
tame
'gentle as a horse' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tame that I've seen before include "Domesticated, gentle" , "Flat" , "Domesticated, not wild" , "Domesticated - dull" , "Broken in" .)