Deterred by riding back with posh chap (3,3)
I believe the answer is:
put off
'deterred' is the definition.
(I've seen this before)
'riding back with posh chap' is the wordplay.
'riding' becomes 'up' (I've seen this in other clues).
'back' is a reversal indicator.
'with' says to put letters next to each other.
'posh chap' becomes 'toff' (I've seen this before).
'up' written backwards gives 'pu'.
'pu'+'toff'='PUT-OFF'
'by' acts as a link.
(Other definitions for put off that I've seen before include "Postpone - disconcert" , "Postpone, repel" , "confuse" , "Defer - deter" , "Defer to a later time, or alienate" .)