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