Star very mature on the rise (4)
I believe the answer is:
vega
'star' is the definition.
(star in the constellation Lyra)
'very mature on the rise' is the wordplay.
'very' becomes 'v' (abbreviation).
'mature' becomes 'age' (synonyms).
'on the rise' says the letters should be written in reverse (in a down clue, letters go up).
'age' backwards is 'ega'.
'v'+'ega'='VEGA'
(Other definitions for vega that I've seen before include "Bright star of constellation Lyra" , "Gave turn to a star" , "Fifth brightest star" , "Brightest star of constellation Lyra" , "Star in Lyra; grassy plain (Spanish)" .)