Posed with one's plant - that's acceptable (12)
I believe the answer is:
satisfactory
'acceptable' is the definition.
('satisfactory' is associated in meaning with 'acceptable')
'posed with one's plant' is the wordplay.
'posed' becomes 'sat' ('sit' can be a synonym of 'pose').
'with' means one lot of letters go next to another.
'one's' becomes 'is' (roman numeral).
'plant' becomes 'factory' (factory is a kind of plant**).
'sat'+'is'+'factory'='SATISFACTORY'
'that's' acts as a link.
(Other definitions for satisfactory that I've seen before include "unexceptional" , "Fulfilling expectations" , "Acceptable - adequate" , "Meeting expectations" , "OK" .)