One who gets by as a horse dealer (5)
I believe the answer is:
coper
This clue is a double definition.
'one who gets by' is the first definition.
(someone who can cope)
'a horse dealer' is the second definition.
(archaic term for a horse dealer)
'as' is the link.
(Other definitions for coper that I've seen before include "who deals with Arabs and others?" , "One managing" , "buy and sell horses?" , "Horse-dealer" , "One who manages to manage" .)