Stewart travelled on horse endlessly (3)
I believe the answer is:
rod
'stewart' is the definition.
(I've seen this in other clues)
'travelled on horse endlessly' is the wordplay.
'travelled on horse' becomes 'rode' (I've seen this before).
'endlessly' means to remove the last letter.
'rode' with its final letter taken off is 'ROD'.
(Other definitions for rod that I've seen before include "Angler's cane" , "Angler's equipment" , "Cane - shaft" , "Long, straight stick" , "Little boy" .)