The core of his endless forced inactivity (8)
I believe the answer is:
idleness
'inactivity' is the definition.
(idleness is a kind of inactivity)
'the core of his endless forced' is the wordplay.
'the core of' says to take the centre.
'forced' indicates anagramming the letters.
The middle letter of 'his' is 'i'.
'endless' with letters rearranged gives 'dleness'.
'i'+'dleness'='IDLENESS'
(Other definitions for idleness that I've seen before include "Lazy quality" , "State of doing nothing" , "Disinclination to work" , "Sloth - there may be love in it" , "Send lies (anag.)" .)