Fly high crossing line of the sun (5)
I believe the answer is:
solar
'of the sun' is the definition.
(solar means relating to the sun)
'fly high crossing line' is the wordplay.
'fly high' becomes 'soar' (soar up into the air).
'crossing' indicates putting letters inside.
'line' becomes 'l' (used when specifying particular lines from a poem).
'soar' enclosing 'l' is 'SOLAR'.
(Other definitions for solar that I've seen before include "revolutionary system" , "Powered by the sun" , "chamber in old house" , "Mediaeval chamber" , "like a star" .)