Powerful ray left sea bream floundering (5,4)
I believe the answer is:
laser beam
'powerful ray' is the definition.
'left sea bream floundering' is the wordplay.
'left' becomes 'l' (common abbreviation).
'floundering' is an anagram indicator.
'sea'+'bream'='seabream'
'seabream' is an anagram of 'aserbeam'.
'l'+'aserbeam'='LASER BEAM'
(Other definitions for laser beam that I've seen before include "Shaft of light or electro-mechanic radiation" , "Column of intensified light" , "penetrating light" , "Intense ray of light" , "Shaft of light or electromagnetic radiation" .)