Clue: Tree choked by parasitic plant in a miserable state (8)
Answer: MOPINESS
Explanation: Definition: a 
An envelope ('choked by') of PINE ('tree') in MOSS ('parasitic plant'). Very possibly we have amongst our number some experts on the bryophytes who can give a authoritative answer, but as a layman, the description 'parasitic plant' seems to me wide of the mark. Chambers does say of mosses "… the zygote grows into a small spore-bearing capsule that grows parasitically on the parent plant", but that strikes me as like saying that a fetus is parasitic on its mother. I would have thought that parasitism only applies when the host is a different species. Even though many mosses grow on trees, they are epiphytic, not parasitic.