Note switches in random mutation? (6)
I believe the answer is:
change
'random mutation?' is the definition.
'change' can be an answer for 'mutation?' (mutation is a kind of change). I'm unsure of the 'random' bit.
'note switches in' is the wordplay.
'note' becomes 'g' (musical note).
'switches' becomes 'chane' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'g' inserted inside 'chane' is 'CHANGE'.
(Other definitions for change that I've seen before include "Coins of low value" , "Cash" , "Alter for small coins" , "Redesign" , "Innovation" .)