Song composed about a rending heart (8)
I believe the answer is:
serenade
'song' is the definition.
(serenade is a kind of song)
'composed about a rending heart' is the wordplay.
'composed' becomes 'serene' (I've seen this before).
'about' is an insertion indicator.
'heart' indicates the central letters.
The centre of 'rending' is 'd'.
'a'+'d'='ad'
'serene' placed around 'ad' is 'SERENADE'.
(Other definitions for serenade that I've seen before include "Sing to (lover)" , "Song sung by suitor" , "Evening music (played outside woman's house)" , "Murder" , "Play music, sing for someone" .)