Weight of notes varied (5)
I believe the answer is:
stone
'weight' is the definition.
(stone is a unit of weight)
'notes varied' is the wordplay.
'varied' indicates an anagram.
'notes' with letters rearranged gives 'STONE'.
'of' acts as a link.
(Other definitions for stone that I've seen before include "that one will 26d at 8a [FLING at HIGHLAND GAMES]" , "Piece of rock - weight" , "See 19" , "Seed of e.g. cherry" , "Kernel" .)