Tree minute but unbending (4)
I believe the answer is:
firm
'unbending' is the definition.
(thesaurus)
'tree minute' is the wordplay.
'tree' becomes 'fir' (fir is a kind of tree**).
'minute' becomes 'm' (abbreviation).
'fir'+'m'='FIRM'
'but' is the link.
(Other definitions for firm that I've seen before include "Steady sort of business organisation" , "corporation?" , "Hard" , "Solid - business" , "Resolute; company" .)