Countermand importing a plant (6)
I believe the answer is:
annual
'plant' is the definition.
(annual is a type of plant)
'countermand importing a' is the wordplay.
'countermand' becomes 'annul' (synonyms).
'importing' is an insertion indicator.
'annul' enclosing 'a' is 'ANNUAL'.
(Other definitions for annual that I've seen before include "Book published yearly" , "Yearly publication" , "Every twelve months" , "Happening once each year" , "Hardy it may be" .)