One's told to heighten beams (4)
I believe the answer is:
rays
'beams' is the definition.
(rays of light)
'one's told to heighten' is the wordplay.
'one's told' indicates a 'sounds like' (homophone) clue.
'to heighten' becomes 'raise' (synonyms).
'raise' is a homophone of 'RAYS'.
(Other definitions for rays that I've seen before include "Straight lines extending from a point" , "Flat fish" , "Sunbeams" , "Beams from sun or moon" , "Beams - fish" .)