Murderer's impertinence when in jails (8)
I believe the answer is:
assassin
'murderer's' is the definition.
(I've seen this before)
'impertinence when in jails' is the wordplay.
'impertinence' becomes 'sass' (sass is a kind of impertinence).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'jails' means one lot of letters goes inside another (inserted letters are kept or jailed).
'as'+'in'='asin'
'sass' going into 'asin' is 'ASSASSIN'.
(Other definitions for assassin that I've seen before include "Hired killer" , "Hired murderer" , "Hitman" , "butcher" , "One making hits" .)