Seal off river in a beam of light (5)
I believe the answer is:
laser
'beam of light' is the definition.
'laser' can be an answer for 'light' (lasers are a type of light). I'm not sure about the 'beam of' bit.
'seal off river' is the wordplay.
'off' indicates anagramming the letters.
'river' becomes 'r' (abbreviation for river).
'seal' with letters rearranged gives 'lase'.
'lase'+'r'='LASER'
'in a' is the link.
Can you help me to learn more?
(Other definitions for laser that I've seen before include "Device producing an intense light beam - kind of sailing dinghy" , "Intense light generator" , "Device that generates an intense beam of light" , "Device yielding powerful beam" , "Powerful light source" .)