Enthusiastic about piebald horse failing to start (4)
I believe the answer is:
into
'enthusiastic about' is the definition.
(into can mean interested in or enthusiastic about)
'piebald horse failing to start' is the wordplay.
'piebald horse' becomes 'pinto' (I've seen this before).
'failing to start' means to remove the first letter.
'pinto' with its initial letter taken off is 'INTO'.
(Other definitions for into that I've seen before include "To a position within" , "Expression of division" , "Keenly interested in" , "Two -- six is three" , "Teachers' trade union" .)