Entirely so, when visiting English island (5,4)
I believe the answer is:
every inch
'entirely' is the definition.
(I've seen this before)
'so when visiting english island' is the wordplay.
'so' becomes 'very' (I've seen this in another clue).
'when visiting' means one lot of letters goes inside another.
'english' becomes 'E' (abbreviation as in OED).
'island' becomes 'inch' (inch can mean a small island).
'e'+'inch'='einch'
'very' placed into 'einch' is 'EVERY INCH'.
(Other definitions for every inch that I've seen before include "Wholly" , "Entirely" , "In all departments" , "Absolutely!" , "completely" .)