Nearly all the conifers are mountain plants (7)
I believe the answer is:
alpines
'mountain plants' is the definition.
(I've seen this before)
'nearly all the conifers' is the wordplay.
'nearly all' becomes 'al'.
'the conifers' becomes 'pines' (pine is a kind of conifer).
'al'+'pines'='ALPINES'
'are' is the link.
(Other definitions for alpines that I've seen before include "Plants in mountains" , "Plants that grow in mountain areas" , "Some plants; spaniel (anag.)" , "Natives of heights" .)