Extremely sessile, like herbaceous plant (6)
I believe the answer is:
sesame
'herbaceous plant' is the definition.
(sesame is a kind of herbaceous plant)
'extremely sessile like' is the wordplay.
'extremely' suggests removing the centre.
'like' becomes 'same' (synonyms).
'sessile' with its middle removed is 'se'.
'se'+'same'='SESAME'
(Other definitions for sesame that I've seen before include "which magically opens" , "Plant yielding seeds used for flavouring bread" , "Plant grown for its seeds" , "said to be effective in opening spell" , "Plant with oil-rich seeds" .)