Beam that’s almost three feet round (3)
I believe the answer is:
'beam' is the definition.
('ray' can be a synonym of 'beam')
'almost three feet round' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'three feet' becomes 'yard' (I've seen this before).
'round' says the letters should be written in reverse.
'yard' with its final letter removed is 'yar'.
'yar' back-to-front is 'RAY'.
'that's' acts as a link.
(Other definitions for ray that I've seen before include "Shaft of light" , "Broad flat fish; beam" , "Gleam" , "Shaft, beam" , "Beam; flat-bodied fish" .)