Like a couch potato (5)
I believe the answer is:
'like a couch potato' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for inert that I've seen before include "Unable to move or resist motion" , "Motionless or lifeless" , "Lacking vigour, inactive" , "Indolent - unreactive" , "Motionless or inanimate" .)