Time to invest in kites — they're seen in the sky (7)
I believe the answer is:
planets
'they're seen in the sky' is the definition.
(planets are seen in the sky)
'time to invest in kites' is the wordplay.
'time' becomes 't' (abbreviation).
'to invest in' means one lot of letters goes inside another.
'kites' becomes 'planes' (kite is a slang term for an aeroplane).
't' put into 'planes' is 'PLANETS'.
(Other definitions for planets that I've seen before include "Ten laps (anag.)" , "Big bodies" , "Heavenly bodies" , "Celestial bodies" .)