Pose as one individual in couple (11)
I believe the answer is:
impersonate
'pose as' is the definition.
(synonyms)
'one individual in couple' is the wordplay.
'one' becomes 'i' (Roman numeral).
'individual' becomes 'person' (synonyms).
'in' is an insertion indicator.
'couple' becomes 'mate' (synonyms).
'person' placed within 'mate' is 'mpersonate'.
'i'+'mpersonate'='IMPERSONATE'
(Other definitions for impersonate that I've seen before include "to be an impressionist" , "Copy" , "Pretend to be (another person)" , "Pinch identity of" , "Mimic" .)