A line couple rejected audibly (5)
I believe the answer is:
aloud
'audibly' is the definition.
(I've seen this in another clue)
'a line couple rejected' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'couple' becomes 'duo' (synonyms).
'rejected' shows that the letters should be reversed in order.
'duo' back-to-front is 'oud'.
'a'+'l'+'oud'='ALOUD'
(Other definitions for aloud that I've seen before include "it is said" , "intended for audience" , "So as to be heard" , "in a normal voice" , "Take part in, link" .)