Quickly as a Parisian ruffian drops an H! (5)
I believe the answer is:
apace
'quickly' is the definition.
(I know that quickly can be written as apace)
'a parisian ruffian drops an h' is the wordplay.
'a parisian ruffian' becomes 'apache' (I've seen this in another clue).
'drops' is a deletion indicator.
'apache' with 'h' taken away is 'APACE'.
'as' acts as a link.
(Other definitions for apace that I've seen before include "flat out" , "at high speed" , "(Move on) swiftly" , "What's quickly" , "At a rapid rate" .)