Horse: might one be on the phone? (7)
I believe the answer is:
charger
I believe this clue is a double definition.
'horse' is the first definition.
(charger is a kind of horse)
'might one be on the phone?' is the second definition.
(a phone charger)
(Other definitions for charger that I've seen before include "A warhorse; a serving-dish" , "could be of use to battery" , "One demanding payment" , "Type of platter" , "One asking money for" .)