Knight, having left English city suddenly (6)
I believe the answer is:
presto
'suddenly' is the definition.
'knight having left english city' is the wordplay.
'knight' becomes 'N' (chess abbreviation).
'having left' suggests deleting specific letters.
'english city' becomes 'preston' (I've seen this before).
'preston' with 'n' removed is 'PRESTO'.
(Other definitions for presto that I've seen before include "(Musically) fast" , "rapidly" , "quick to score" , "Hey, it's a magician's word" , "Very fast (mus.)" .)