Evidence I’ve altered implicating King’s Head music man (9)
I believe the answer is:
prokofiev
'music man' is the definition.
(Sergei Prokofiev)
'evidence i've altered implicating king's head' is the wordplay.
'evidence' becomes 'proof' (proof is a kind of evidence**).
'altered' is an anagram indicator.
'implicating' indicates putting letters inside.
'head' suggests taking the first letters.
The first letter of 'kings' is 'k'.
'ive' is an anagram of 'iev'.
'proof'+'iev'='proofiev'
'proofiev' enclosing 'k' is 'PROKOFIEV'.
(Another definition for prokofiev that I've seen is " Russian composer (Peter and the Wolf)".)