Claim horrible, large-scale cars will be rejected (6)
I believe the answer is:
allege
'claim' is the definition.
(to allege something is to make a claim)
'horrible large-scale cars will be rejected' is the wordplay.
'horrible' indicates an anagram.
'will be rejected' is a deletion indicator.
'large'+'scale'='largescale'
'largescale' with 'cars' removed is 'lgeale'.
'lgeale' with letters rearranged gives 'ALLEGE'.
(Other definitions for allege that I've seen before include "Make a claim without proof" , "Declare or assert without proof" , "Assert, state without proof" , "Claim (a person is guilty)" , "Cite in discussion" .)