Performer on horseback in show (4,2)
I believe the answer is:
turn up
'show' is the definition.
(I've seen this before)
'performer on horseback' is the wordplay.
'performer' becomes 'turn' ('turn' can mean a performer as well as their act**).
'on horseback' becomes 'up'.
'turn'+'up'='TURN-UP'
'in' is the link.
(Other definitions for turn up that I've seen before include "surprise" , "make louder" , "part of leg" , "put in an appearance" , "Arrive (uninvited?)" .)