Friend about to sing (4)
I believe the answer is:
chum
'friend' is the definition.
(I know that chum is a type of friend)
'about to sing' is the wordplay.
'about' becomes 'c' (abbreviation for circa).
'to sing' becomes 'hum' (humming is a kind of singing).
'c'+'hum'='CHUM'
(Other definitions for chum that I've seen before include "Pal, mate" , "Good friend" , "Salmon" , "Close friend (colloq.)" , "Pal, buddy" .)