Undoubtedly like one to visit cathedral city (6)
I believe the answer is:
easily
'undoubtedly' is the definition.
(thesaurus)
'like one to visit cathedral city' is the wordplay.
'like' becomes 'as'.
'one' becomes 'i' (roman numeral).
'to visit' is an insertion indicator.
'cathedral city' becomes 'ely' (I've seen this before).
'as'+'i'='asi'
'asi' put within 'ely' is 'EASILY'.
(Other definitions for easily that I've seen before include "Comfortably" , "no problem!" , "Without difficulty or effort" , "without doubt" , "With little difficulty" .)