In Germany I must cross northern island (4)
I believe the answer is:
'island' is the definition.
(inch can mean a small island)
'in germany i must cross northern' is the wordplay.
'in germany i' becomes 'ich' ('I' in German).
'must cross' indicates putting letters inside.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'ich' placed around 'n' is 'INCH'.
(Other definitions for inch that I've seen before include "Creep, edge" , "Move forward slowly and carefully" , "One twelfth of a foot" , "0.08333 of a foot" , "1/12 of foot" .)