Parisian gangster quickly seizing hostage at the outset (6)
I believe the answer is:
apache
'parisian gangster' is the definition.
(I've seen this before)
'quickly seizing hostage at the outset' is the wordplay.
'quickly' becomes 'apace' (synonyms).
'seizing' indicates putting letters inside (inserted letters are seized).
'at the outset' indicates taking the first letters (the outset is the beginning).
The initial letter of 'hostage' is 'h'.
'apace' enclosing 'h' is 'APACHE'.
(Other definitions for apache that I've seen before include "language" , "Hooligan in Paris" , "ruffian in capital of France" , "Member of a native American people" , "American speech" .)