Understanding popular quote, say (7)
I believe the answer is:
insight
'understanding' is the definition.
(I know that insight is a type of understanding)
'popular quote say' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'quote' becomes 'cite' (synonyms).
'say' shows a homophone (sound like).
'cite' sounds like 'sight'.
'in'+'sight'='INSIGHT'
(Other definitions for insight that I've seen before include "ability to perceive clearly" , "Sudden understanding" , "Enlightenment" , "famous Sunday Times team" , "Furniture from Princes Street for" .)