Weighing device in proportion (5)
I believe the answer is:
scale
This clue is a double definition.
'weighing device' is the first definition.
(I've seen this in another clue)
'proportion' is the second definition.
(I know that scale is a type of proportion)
'in' is the link.
(Other definitions for scale that I've seen before include "Hierarchy" , "Horny plate; balance" , "Doh, re, me etc" , "Relative size (of something)" , "Graduated table" .)