No weight is unacceptable (3,2)
I believe the answer is:
not on
'unacceptable' is the definition.
(something 'not on' is unacceptable or frowned upon)
'no weight' is the wordplay.
'weight' becomes 'ton' (unit of weight).
'no'+'ton'='NOT ON'
'is' acts as a link.
(Other definitions for not on that I've seen before include "Common expression meaning won't be entertained or tolerated" , "Unacceptable (colloq.)" , "It's unfeasible" , "Informally, cannot be permitted or tolerated" , "Out of the question" .)