Vivacious horse going over yard (6)
I believe the answer is:
punchy
'vivacious' is the definition.
(both can mean animated or powerful)
'horse going over yard' is the wordplay.
'horse' becomes 'punch' (type of horse).
'going over yard' becomes 'y' (I can't justify this - if you can you should give a lot more credence to this answer).
'punch'+'y'='PUNCHY'
(Other definitions for punchy that I've seen before include "Sharp and powerful" , "Incisive" , "Vigorous (in speech)" , "Forceful" , "like a former magazine?" .)