Sunshine, essentially (4)
I believe the answer is:
rays
'sunshine essentially' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rays that I've seen before include "Columns of light or heat" , "we hope to catch occasionally out of the blue?" , "Fish with wing-shaped fins and long tails" , "shafts" , "Beams from sun or moon" .)