Unusual find full of seldom-encountered radiation (8)
I believe the answer is:
infrared
'radiation' is the definition.
(infrared is a kind of radiation)
'unusual find full of seldom-encountered' is the wordplay.
'unusual' indicates anagramming the letters.
'full of' is an insertion indicator.
'seldom encountered' becomes 'rare' (I've seen this before).
'find' with letters rearranged gives 'infd'.
'infd' going around 'rare' is 'INFRARED'.
(Other definitions for infrared that I've seen before include "sort of photography" , "Fan drier (anag) - type of radiation" , "With a frequency between the red end of the visible spectrum and microwaves" , "Kind of rays that enable one to see in the dark" , "heat" .)