Unknown person capturing liberal assassin (9)
I believe the answer is:
strangler
'assassin' is the definition.
(thesaurus)
'unknown person capturing liberal' is the wordplay.
'unknown person' becomes 'stranger' (I've seen this before).
'capturing' means one lot of letters goes inside another.
'liberal' becomes 'l' (abbreviation).
'stranger' going around 'l' is 'STRANGLER'.
(Other definitions for strangler that I've seen before include "Killer by choking" , "he could take your breath away" , "criminal" , "Killer compressing the windpipe" , "Murderer" .)