When one enters Ely comfortably (6)
I believe the answer is:
easily
'comfortably' is the definition.
(I've seen this before)
'when one enters ely' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'one' becomes 'i' (Roman numeral).
'enters' is an insertion indicator.
'as'+'i'='asi'
'asi' placed within 'ely' is 'EASILY'.
(Other definitions for easily that I've seen before include "no problem!" , "Comfortably" , "Simply" , "Very probably" , "Effortlessly" .)