Not up to weight? (3)
I believe the answer is:
ton
'weight' is the definition.
(unit of weight)
'not up' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'not' backwards is 'TON'.
'to' acts as a link.
(Other definitions for ton that I've seen before include "20 cwt" , "100 mph, runs, etc" , "People of fashion" , "2240lbs" , "Style" .)