Concrete mounts securing small, light device (5)
I believe the answer is:
laser
'light device' is the definition.
'laser' can be an answer for 'device' (laser is a kind of device). I'm unsure of the 'light' bit.
'concrete mounts securing small' is the wordplay.
'concrete' becomes 'real' (associated in meaning).
'mounts' shows that the letters should be reversed in order (I've seen 'mounted' mean this).
'securing' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'real' written backwards gives 'laer'.
'laer' placed around 's' is 'LASER'.
Can you help me to learn more?
(Other definitions for laser that I've seen before include "Coherent light beam" , "Device yielding powerful beam" , "Device producing an intense beam of light or radiation" , "Intense light generator" , "modern light industry" .)