Cheerful message received: it's a flag (5,5)
I believe the answer is:
jolly roger
'a flag' is the definition.
(Jolly Roger is an example)
'cheerful message received' is the wordplay.
'cheerful' becomes 'jolly' (I've seen this in another clue).
'message received' becomes 'roger ' (radio communications).
'jolly'+'roger'='JOLLY ROGER'
'it's' is the link.
(Other definitions for jolly roger that I've seen before include "In the main a fearful sight" , "standard for boarders?" , "Pirate flag" , "Pirates' flag" , "standard not welcomed in the main" .)