Daisy (5)
'daisy' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for aster that I've seen before include "Flower of the daisy family - rates (anag)" , "Tears (anag) - plant" , "Christmas daisy" , "eg Michaelmas daisy" , "Plant like a Michaelmas daisy" .)