Nasty old boy turning violent in old constituency (6,7)
I believe the answer is:
rotten borough
'old constituency' is the definition.
(I've seen this before)
'nasty old boy turning violent' is the wordplay.
'nasty' becomes 'rotten' (I've seen this before**).
'old boy' becomes 'ob'.
'turning' says the letters should be written backwards.
'violent' becomes 'rough'.
'ob' back-to-front is 'bo'.
'rotten'+'bo'+'rough'='ROTTEN BOROUGH'
'in' acts as a link.
(Other definitions for rotten borough that I've seen before include "old constituency" , "Corrupt constituency in former times" , "English parliamentary constituency of a kind abolished by the 1832 Reform Act" , "Tiny-electorate seat (hist.)" .)