Overheard, a beautiful thing (6)
I believe the answer is:
affair
'thing' is the definition.
(I know that thing can be written as affair)
'overheard a beautiful' is the wordplay.
'overheard' becomes 'af' (I can't justify this - if you can you should believe this answer much more).
'a beautiful' becomes 'fair' (I've seen this before).
'af'+'fair'='AFFAIR'
Can you help me to learn more?
(Other definitions for affair that I've seen before include "A fling (or something more serious?)" , "Liaison or intrigue" , "Secret liaison" , "Illicit liaison" , "Matter or concern, perhaps of the heart" .)