In turn I've become idle (8)
I believe the answer is:
inactive
'idle' is the definition.
(similar in meaning)
'in turn i've' is the wordplay.
'turn' becomes 'act' (theatre).
'in'+'act'+'ive'='INACTIVE'
'become' acts as a link.
(Other definitions for inactive that I've seen before include "Chemically inert" , "sluggish" , "Passive or idle" , "Idle, showing no movement" , "Not doing anything, passive" .)