Weight not brought up (3)
I believe the answer is:
ton
'weight' is the definition.
(unit of weight)
'not brought up' is the wordplay.
'brought up' is a reversal indicator (in a down clue, letters go up).
'not' reversed gives 'TON'.
(Other definitions for ton that I've seen before include "Two thousand, two hundred and forty lbs avoirdupois" , "1000 kilos" , "Twenty cwt" , "2240 Ibs" , "Century" .)