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