Without difficulty, see while one is inside (6)
I believe the answer is:
easily
'without difficulty' is the definition.
('easily' can be a synonym of 'without difficulty')
'see while one is inside' is the wordplay.
'see' becomes 'ely' (example where cathedral located in diocese).
'while' becomes 'as'.
'one' becomes 'i' (Roman numeral).
'is inside' indicates putting letters inside.
'as'+'i'='asi'
'ely' going around 'asi' is 'EASILY'.
(Other definitions for easily that I've seen before include "Comfortably" , "Effortlessly" , "no problem!" , "Without much difficulty" , "Very probably" .)