Note single skull, say (4)
I believe the answer is:
bone
'skull say' is the definition.
(I know that skull is a type of bone)
'note single' is the wordplay.
'note' becomes 'b' (musical note).
'single' becomes 'one' (a single thing - only one).
'b'+'one'='BONE'
(Other definitions for bone that I've seen before include "Scapula, radius or tibia?" , "It makes up the skeleton" , "Skeleton component" , "#Close to it is risque" , "Eg, rib" .)