Old Republican, Romney halved weight in prison accommodation (8)
I believe the answer is:
cromwell
'old republican' is the definition.
(anti-monarchy leader in English Civil War)
'romney halved weight in prison accommodation' is the wordplay.
'halved' means to take half the letters.
'weight' becomes 'w' (short for weight).
'in' means one lot of letters goes inside another.
'prison accommodation' becomes 'cell' (I've seen this before).
'romney' with half the letters taken is 'rom'.
'rom'+'w'='romw'
'romw' inserted inside 'cell' is 'CROMWELL'.
(Other definitions for cromwell that I've seen before include "Lord Protector (sixteen fifty three to sixteen fifty eight)" , "Oliver, Puritan leader during 17th century English Civil War" , "Leader of the Roundheads, d.1658" , "English general" , "Old English statesman" .)