Terribly mean individual in plant (7)
I believe the answer is:
anemone
'plant' is the definition.
(anemone is a kind of plant)
'terribly mean individual' is the wordplay.
'terribly' indicates anagramming the letters.
'individual' becomes 'one' (an individual is one person).
'mean' anagrammed gives 'anem'.
'anem'+'one'='ANEMONE'
'in' is the link.
(Other definitions for anemone that I've seen before include "Plant; marine creature" , "Plant with brightly coloured flowers - found also in sea" , "bloom" , "climbing plant" , "bloomer" .)