Qualifier raved about bad start (6)
I believe the answer is:
adverb
'qualifier' is the definition.
(adverb is a kind of qualifier)
'raved about bad start' is the wordplay.
'about' is an anagram indicator.
'start' says to take the initial letters.
The initial letter of 'bad' is 'b'.
'raved' is an anagram of 'adver'.
'adver'+'b'='ADVERB'
(Other definitions for adverb that I've seen before include "Author of The Thirty-Nine Steps" , "Perhaps stupidly" , "twelve for example" , "Qualifying word (gram.)" , "It qualifies the predicate in a sentence" .)