Boy put on horse (4)
I believe the answer is:
seth
'boy' is the definition.
(I've seen this before)
'put on horse' is the wordplay.
'put' becomes 'set' (setting is a kind of putting**).
'on' is a charade indicator (letters next to each other).
'horse' becomes 'h' ('H' can be a synonym of 'horse').
'set'+'h'='SETH'
(Other definitions for seth that I've seen before include "Boy's name" , "Early man" , "Indian novelist, Vikram" , "Indian writer" , "One of the first names recorded" .)