Quietly enthusiastic about horse (5)
I believe the answer is:
pinto
'horse' is the definition.
(pinto is a kind of horse)
'quietly enthusiastic' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'enthusiastic' becomes 'into' (I've seen this in other clues**).
'p'+'into'='PINTO'
'about' acts as a link.
(Other definitions for pinto that I've seen before include "H [HORSE]" , "Mount" , "Piebald horse (US)" , "A piebald pony" , "Spotted or piebald horse" .)