Star's very long time rising (4)
I believe the answer is:
vega
'star's' is the definition.
(star in the constellation Lyra)
'very long time rising' is the wordplay.
'very' becomes 'v' (common abbreviation).
'long time' becomes 'age' (an age is a period of time).
'rising' says the letters should be written in reverse (letters written upwards in a down clue).
'age' reversed gives 'ega'.
'v'+'ega'='VEGA'
(Other definitions for vega that I've seen before include "Star in Lyra; grassy plain (Spanish)" , "Brightest star of constellation Lyra" , "heavenly body" , "Fifth brightest star" , "Bright star of constellation Lyra" .)