Notes about weight (5)
I believe the answer is:
stone
'weight' is the definition.
(I've seen this before)
'notes about' is the wordplay.
'about' indicates anagramming the letters.
'notes' is an anagram of 'STONE'.
(Other definitions for stone that I've seen before include "Pebble" , "See 5" , "Boulder" , "Rock; hard seed" , "Seed of e.g. cherry" .)