Florist's unit (4)
I believe the answer is:
stem
'florist's unit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for stem that I've seen before include "Shoot; arrest" , "Stop or slow the flow of" , "Block" , "Main central part as of plant or glass" , "Main part of noun or verb" .)