Wild clematis (3,4,5)
I believe the answer is:
old mans beard
'wild clematis' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for old mans beard that I've seen before include "in which eight birds nested according to Lear" , "Wild clematis" , "Trailing plant" , "growing on craggy face, perhaps" , "that grows up fast" .)