Separating from popular duo usually testing (7)
I believe the answer is:
arduous
'testing' is the definition.
'separating from popular duo usually' is the wordplay.
'separating from' says the answer is hidden in the clue (the answer must be separated out of the clue).
'ARDUOUS' can be found hidden inside 'popular duo usually'.
(Other definitions for arduous that I've seen before include "Requiring much effort" , "Extremely taxing" , "Burdensome" , "Requiring great effort" , "Requiring hard work" .)