Gold trees, terribly plain (7)
I believe the answer is:
austere
'plain' is the definition.
(similar in meaning)
'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 "Severe; lacking luxuries" , "Plain, cold" , "severely self-disciplined" , "Stern, grave" , "Without luxury or ornament" .)