Come by tramp, having spotted horse (4,4)
I believe the answer is:
bump into
'come' is the definition.
(thesaurus)
'tramp having spotted horse' is the wordplay.
'tramp' becomes 'bum' ('bum' can be a synonym of 'tramp').
'having' is a charade indicator (letters next to each other).
'spotted horse' becomes 'pinto' (I've seen this before).
'bum'+'pinto'='BUMP INTO'
'by' is the link.
(Other definitions for bump into that I've seen before include "meet, unexpectedly" , "Chance upon" , "Happen to meet - ouch!" , "Meet by chance - in collision?" , "Unintentionally come across" .)