Sign of diminution (5)
I believe the answer is:
minus
'sign' is the definition.
(I know that minus is a type of sign)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for minus that I've seen before include "Disadvantage" , "Negative sign - negative thing" , "'Take away' sign" , "Lacking; below zero" , "Take-away (sign)" .)