Memorable quotation on debut is questionable (5,4)
I believe the answer is:
sound bite
'memorable quotation' is the definition.
(a soundbite is a memorable or succinct quotation)
'on debut is questionable' is the wordplay.
'questionable' is an anagram indicator.
'on'+'debut'+'is'='ondebutis'
'ondebutis' is an anagram of 'SOUND BITE'.
(Other definitions for sound bite that I've seen before include "Newsworthy quote" , "few words" , "No sub-edit for short snappy extract from interview or speech" , "No subedit for short snappy extract from speech as on TV" , "brief comment heard on the news?" .)