Somehow seal the top of radio light amplifier (5)
I believe the answer is:
laser
'light amplifier' is the definition.
(I've seen this in another clue)
'somehow seal the top of radio' is the wordplay.
'somehow' indicates an anagram.
'the top of' says to take the initial letters.
The first letter of 'radio' is 'r'.
'seal' with letters rearranged gives 'lase'.
'lase'+'r'='LASER'
(Other definitions for laser that I've seen before include "Optical device producing an intense beam of light" , "Surgical device" , "E.g. surgical light-beam" , "Generator of beam" , "Device that generates an intense beam of light" .)