Star having time in a retreat (6)
I believe the answer is:
'star' is the definition.
(Altair is a star in the constellation of Aquila)
'time in a retreat' is the wordplay.
'time' becomes 't' (abbreviation).
'in' means one lot of letters goes inside another.
'retreat' becomes 'lair' (both can mean a private hideout).
't' put into 'alair' is 'ALTAIR'.
'having' is the link.
(Other definitions for altair that I've seen before include "Bright star in Aquila" , "A trial (anag) - star" , "First-magnitude star" , "Lariat (anag) - star in Aquila" , "Member of Eagle family" .)