Note name assumed by wild alpine plant (9)
I believe the answer is:
perennial
'plant' is the definition.
(I know that perennial is a type of plant)
'note name assumed by wild alpine' is the wordplay.
'note' becomes 're' (as in do-re-mi).
'name' becomes 'n' (common abbreviation e.g. on forms).
'assumed by' means one lot of letters goes inside another (inserted letters are absorbed or assumed by others).
'wild alpine' becomes 'penial' (I can't explain this - if you can you should give a lot more credence to this answer).
're'+'n'='ren'
'ren' going into 'penial' is 'PERENNIAL'.
(Other definitions for perennial that I've seen before include "Enduring" , "Plant that lives for a number of years" , "Plant that lives for many years" , "Everlasting" , "Constant" .)