Small note about weight (5)
I believe the answer is:
stone
'weight' is the definition.
(stone is a unit of weight)
'small note about' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'about' indicates anagramming the letters.
'note' anagrammed gives 'tone'.
's'+'tone'='STONE'
(Other definitions for stone that I've seen before include "that one will 26d at 8a [FLING at HIGHLAND GAMES]" , "Rag" , "Imperial unit of weight" , "See 21" , "Little rock" .)