Sue quaintly pleasing, ordinary language first (9)
I believe the answer is:
prosecute
'sue' is the definition.
(I've seen this before)
'quaintly pleasing ordinary language first' is the wordplay.
'quaintly pleasing' becomes 'cute' (I've seen this in other clues).
'ordinary language' becomes 'prose' (I've seen this before).
'first' is a charade indicator (letters next to each other).
'cute' after 'prose' is 'PROSECUTE'.
(Other definitions for prosecute that I've seen before include "Practise" , "Sue" , "Bring a criminal action against" , "Pursue" , "Represent the Crown in court" .)