A Royal Marines orchestra’s form of identification (7)
I believe the answer is:
armband
'form of identification' is the definition.
(I've seen this before)
'a royal marines orchestra's' is the wordplay.
'royal marines' becomes 'rm'.
'orchestra' becomes 'band' (I've seen this before).
'a'+'rm'+'band'='ARMBAND'
(Other definitions for armband that I've seen before include "It signals captain" , "What might hold up sleeve" , "Something worn around sleeve" , "Worn by football captain or mourner" , "Swimming aid" .)