Composer having king amongst supporters (6)
I believe the answer is:
brahms
'composer' is the definition.
(I know that Brahms is a composer)
'king amongst supporters' is the wordplay.
'king' becomes 'HM' (short for His Majesty).
'amongst' indicates putting letters inside (I've seen this in other clues).
'supporters' becomes 'bras' (a bra gives support to breasts).
'hm' put into 'bras' is 'BRAHMS'.
'having' is the link.
(Other definitions for brahms that I've seen before include "He scored" , "German barman" , "'Bah, Mrs, he's a composer (6)'" , "19th century German composer, known for Lullaby" , "German Romantic composer" .)