Take flight, holding line of the sun (5)
I believe the answer is:
solar
'of the sun' is the definition.
(solar means relating to the sun)
'take flight holding line' is the wordplay.
'take flight' becomes 'soar' (to soar is to fly).
'holding' means one lot of letters goes inside another.
'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 "Sun-powered" , "chamber in old house" , "revolutionary system" , "Concerning the sun" , "Relating to our star" .)