Popular single, new, initially thought rude (8)
I believe the answer is:
insolent
'rude' is the definition.
(I've seen this in another clue)
'popular single new initially thought' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'single' becomes 'sole' (similar in meaning).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'initially' indicates taking the first letters.
The initial letter of 'thought' is 't'.
'in'+'sole'+'n'+'t'='INSOLENT'
(Other definitions for insolent that I've seen before include "Showing an arrogant lack of respect" , "Rude and disrespectful" , "Cheeky" , "Bold" , "Rude, showing lack of respect" .)