Mount plate (5)
I believe the answer is:
scale
I believe this clue is a double definition.
'mount' is the first definition.
(I know that scale is a type of mount)
'plate' is the second definition.
(I know that plate can be written as scale)
(Other definitions for scale that I've seen before include "Ratio of, say, map to reality" , "Tones rising maybe" , "Series of notes in an octave" , "Doh, re, me etc" , "Hierarchy" .)