Obliged to be unnatural (6)
I believe the answer is:
forced
I believe this is a double definition.
'obliged' is the first definition.
(forcing is a kind of obliging)
'unnatural' is the second definition.
(similar in meaning)
'to be' acts as a link.
(Other definitions for forced that I've seen before include "Compelled or strained" , "Not spontaneous" , "Compelled or obliged" , "Not natural" , "compulsory" .)