Companion introduced to right sort of plant (6)
I believe the answer is:
lichen
'sort of plant' is the definition.
'companion introduced to right' is the wordplay.
'companion' becomes 'ch' (Companion of Honour).
'introduced to' indicates putting letters inside.
'right' becomes 'lien' (legal right).
'ch' going within 'lien' is 'LICHEN'.
(Other definitions for lichen that I've seen before include "It forms tufts on stones" , "Small plants found on tree-trunks and stones" , "It grows on rocks and trees" , "Tiny plants found on rocks, tree-trunks etc" , "Tiny plants found on rocks and trees" .)