Unnatural, artificial (6)
I believe the answer is:
forced
This clue is a double definition.
'unnatural' is the first definition.
(similar in meaning)
'artificial' is the second definition.
(thesaurus)
(Other definitions for forced that I've seen before include "Compelled, made to obey" , "Compelled or strained" , "Compelled or obliged" , "Compelled, made to do something" , "Not natural" .)