Without doubt getting round the city is fast (8)
I believe the answer is:
securely
'is fast' is the definition.
(thesaurus)
'without doubt getting round the city' is the wordplay.
'without doubt' becomes 'surely' (I've seen this in another clue).
'getting round' is an insertion indicator.
'the city' becomes 'EC' (the City of London postcode).
'surely' going around 'ec' is 'SECURELY'.
(Other definitions for securely that I've seen before include "In a stable way" , "out of danger" , "Firmly (fixed)" , "In a safe manner" .)