Extremist prisoner is embroiled in true act of revenge (8)
I believe the answer is:
reprisal
'act of revenge' is the definition.
(I've seen this before)
'extremist prisoner is embroiled in true' is the wordplay.
'extremist' suggests removing the centre (I've seen 'extremists' mean this).
'embroiled in' is an insertion indicator.
'true' becomes 'real' (similar in meaning).
'prisoner' with its middle removed is 'pr'.
'pr'+'is'='pris'
'pris' placed into 'real' is 'REPRISAL'.
(Other definitions for reprisal that I've seen before include "Retaliatory deed" , "Act of retaliation or vengeance" , "comeback" , "Retaliation for an injury" , "hitting back" .)