Wait, as groom may have to do? (4,4,6)
I believe the answer is:
hold your horses
'wait' is the definition.
(thesaurus)
'groom may have to do?' is the wordplay.
I cannot really see how this works, but
'have' could be 'hold' (holding is a kind of having) and 'hold' is located in the answer.
This may be the basis of clue (or it may be nonsense).
'as' acts as a link.
(Other definitions for hold your horses that I've seen before include "wait a moment" , "The groom may" , "Not so fast" .)