Tree on top of mountain is unshakeable (4)
I believe the answer is:
firm
'unshakeable' is the definition.
(thesaurus)
'tree on top of mountain' is the wordplay.
'tree' becomes 'fir' (fir tree is a kind of tree).
'on top of' suggests taking the first letters.
The first letter of 'mountain' is 'm'.
'fir'+'m'='FIRM'
'is' acts as a link.
(Other definitions for firm that I've seen before include "unlikely to collapse" , "Company - stable" , "Steady and resolute" , "Steadfast" , "solid structure" .)