Plant, one only half coming up (4)
I believe the answer is:
iris
'plant' is the definition.
(type of flower)
'one only half coming up' is the wordplay.
'one' becomes 'i' (Roman numeral).
'only half' suggests halving the letters.
'coming up' becomes 'rising' ('rise' can be a synonym of 'come up').
'rising' with half the letters taken is 'ris'.
'i'+'ris'='IRIS'
(Other definitions for iris that I've seen before include "Eye part or pretty flower" , "Fleur-de-lys" , "who keeps pupil in?" , "Camera diaphragm" , "Rainbow-goddess" .)