Ivy for one always jealous (9)
I believe the answer is:
evergreen
'ivy' is the definition.
(I've seen this before)
'one always jealous' is the wordplay.
'one always' becomes 'ever' (I've seen this before).
'jealous' becomes 'green'.
'ever'+'green'='EVERGREEN'
'for' acts as a link.
(Other definitions for evergreen that I've seen before include "Pine, for example" , "colourful" , "Star anise, say" , "The foliage of the trees remains throughout the year" , "Not dropping leaves" .)