It's not about weight (3)
I believe the answer is:
ton
'weight' is the definition.
(ton is a kind of weight)
'it's not about' is the wordplay.
'it' becomes 't' (abbreviation. e.g. in 'tis**).
'not' becomes 'no' (I've seen this in other clues).
'about' says the letters should be written in reverse.
'no' written backwards gives 'on'.
't'+'on'='TON'
(Other definitions for ton that I've seen before include "2240 Ibs" , "A slang 100mph - heavy" , "20cwt" , "2240lbs" , "Twenty cwt" .)