Gold trees - terribly forbidding (7)
I believe the answer is:
austere
'forbidding' is the definition.
('austere' can be a synonym of 'forbidding')
'gold trees terribly' is the wordplay.
'gold' becomes 'au' (Au is the chemical symbol for gold).
'terribly' indicates anagramming the letters.
'trees' anagrammed gives 'stere'.
'au'+'stere'='AUSTERE'
(Other definitions for austere that I've seen before include "Abstemious and grave" , "Severe or strict in attitude" , "Severe in appearance" , "'Spartan, simple (7)'" , "Severe, ascetic" .)