Reach politician while visiting island (7)
I believe the answer is:
compass
'reach' is the definition.
(I know that reach can be written as compass)
'politician while visiting island' is the wordplay.
'politician' becomes 'MP' (Member of Parliament).
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'visiting' means one lot of letters goes inside another.
'island' becomes 'Cos' (Greek island).
'mp'+'as'='mpas'
'mpas' inserted into 'cos' is 'COMPASS'.
(Other definitions for compass that I've seen before include "may be boxed" , "Range - direction finder" , "Device with magnetic needle for determining direction" , "Range, area" , "Navigational instrument" .)