One rare island plant (4)
I believe the answer is:
iris
'plant' is the definition.
(type of flower)
'one rare island' is the wordplay.
'one' becomes 'i' (Roman numeral).
'rare' becomes 'r' (listed by Oxford Dictionary of Abbreviations).
'island' becomes 'is' (abbreviation).
'i'+'r'+'is'='IRIS'
(Other definitions for iris that I've seen before include "Goddess of the rainbow in the eye" , "Lily" , "Flowers in the eye?" , "Fleur-de-lys" , "plant with showy flowers" .)