Inert yet works forever (8)
I believe the answer is:
eternity
'forever' is the definition.
(synonyms)
'inert yet works' is the wordplay.
'works' indicates anagramming the letters (work can mean to twist or convulse).
'inert'+'yet'='inertyet'
'inertyet' is an anagram of 'ETERNITY'.
(Other definitions for eternity that I've seen before include "That's a very long time" , "You never come to the end of this" , "Blake innocently found it in an hour" , "World without end" , "Unending time" .)