Groaned when heard line-up (4)
I believe the answer is:
side
'line-up' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe there's an association between them I don't understand?
'groaned when heard' is the wordplay.
'groaned' becomes 'sighed' (I've seen this before).
'when heard' shows a homophone (sound like).
'sighed' sounds like 'SIDE'.
Can you help me to learn more?
(Other definitions for side that I've seen before include "Accompanying" , "Airs and graces" , "Minor" , "Team - pretentious manner" , "Competing players" .)