Tree that’s not quite stable (3)
I believe the answer is:
fir
'tree' is the definition.
(fir is a kind of tree)
'not quite stable' is the wordplay.
'not quite' means to remove the last letter.
'stable' becomes 'firm' (similar in meaning).
'firm' with its final letter taken away is 'FIR'.
'that's' acts as a link.
(Other definitions for fir that I've seen before include "Tree for Irishmen" , "Kind of cone-bearing tree" , "Conifer tree" , "Evergreen conifer" , "Coniferous tree" .)