Completely eradicating foremost seasonal plant (5)
I believe the answer is:
holly
'seasonal plant' is the definition.
(plant associated with winter)
'completely eradicating foremost' is the wordplay.
'completely' becomes 'wholly' (synonyms).
'eradicating foremost' means to remove the first letter (the foremost letter is eradicated).
'wholly' with its first letter taken off is 'HOLLY'.
(Other definitions for holly that I've seen before include "Prickly tree" , "Shrub with red berries" , "Plant used for Christmas decorations" , "Festive berry" , "Best Actress Oscar winner 1993, for 'The Piano'" .)