Small number put on weight? Unacceptable! (3,2)
I believe the answer is:
not on
'unacceptable' is the definition.
(something 'not on' is unacceptable or frowned upon)
'small number put on weight?' is the wordplay.
'small number' becomes 'no' (abbreviation for 'number').
'put on' says to put letters next to each other.
'weight?' becomes 'ton' (unit of weight).
'no'+'ton'='NOT ON'
(Other definitions for not on that I've seen before include "Modern usage for unacceptable" , "Modern phrase for unacceptable" , "Out of the question" , "It's unfeasible" , "Informally, can't be accepted or tolerated" .)