Plant, one supported by stake briefly (4)
I believe the answer is:
iris
'plant' is the definition.
(type of flower)
'one supported by stake briefly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'supported by' says to put letters next to each other (in a down clue, some letters hold up others).
'stake' becomes 'risk' (at risk means at stake).
'briefly' means to remove the last letter.
'risk' with its final letter removed is 'ris'.
'i'+'ris'='IRIS'
(Other definitions for iris that I've seen before include "Goddess of the rainbow in the eye" , "bloomer?" , "bit of a looker" , "Flower and eye membrane" , "who keeps pupil in?" .)