It measures radiation from reflective paste to frighten guards (9)
I believe the answer is:
dosimeter
'it measures radiation' is the definition.
The definition suggests a singular noun which matches the answer.
'reflective paste to frighten guards' is the wordplay.
'reflective' is a reversal indicator.
'paste' becomes 'miso' (I've seen this before).
'to frighten' becomes 'deter' (I am not sure about this - if you are sure you should believe this answer much more).
'guards' is an insertion indicator (inserted letters are guarded).
'miso' reversed gives 'osim'.
'osim' placed into 'deter' is 'DOSIMETER'.
'from' acts as a link.
Can you help me to learn more?
(Other definitions for dosimeter that I've seen before include "Device measuring absorbed radiation" , "Measure of absorption" .)