Come across useless type of horse (4,4)
I believe the answer is:
bump into
'come across' is the definition.
(synonyms)
'useless type of horse' is the wordplay.
'useless' becomes 'bum' (I've seen this in other clues).
'type of horse' becomes 'pinto' ().
'bum'+'pinto'='BUMP INTO'
(Other definitions for bump into that I've seen before include "Come across unexpectedly" , "Happen to meet - ouch!" , "Meet by chance - in collision?" , "Encounter" , "Chance upon" .)