Performer in cart is terrific (6)
I believe the answer is:
artist
'performer' is the definition.
(I've seen this before)
'in cart is terrific' is the wordplay.
'in' indicates a hidden word.
'ARTIST' can be found hidden inside 'cart is terrific'.
(Other definitions for artist that I've seen before include "Creator of aesthetic works" , "Model employer, perhaps" , "Painter in dire strait" , "Constable possibly" , "One skilled in creative works" .)