Horrified by experiences in a high-speed car (6)
I believe the answer is:
'horrified' is the definition.
(I know that horrified can be written as aghast)
'experiences in a high-speed car' is the wordplay.
'experiences' becomes 'has' (have can mean to experience or go through).
'in' means one lot of letters goes inside another.
'high-speed car' becomes 'GT' (grand tourer).
'has' going into 'agt' is 'AGHAST'.
'by' acts as a link.
(Other definitions for aghast that I've seen before include "What's carried in bag has teacher" , "Horror-stricken, filled with consternation" , "in awe" , "Taken aback with horror" , "Confounded" .)