Void filled by a plant (6)
I believe the answer is:
annual
'plant' is the definition.
(annual is a type of plant)
'void filled by a' is the wordplay.
'void' becomes 'annul' (synonyms).
'filled by' means one lot of letters goes inside another.
'annul' enclosing 'a' is 'ANNUAL'.
(Other definitions for annual that I've seen before include "Occuring once a year" , "Once a year, perhaps a book" , "Twelve-monthly" , "Plant; yearbook" , "Reference book that's published every year" .)