One facilitates English navy seizing island on way north (7)
I believe the answer is:
enabler
'one facilitates' is the definition.
(someone who enables or facilitates something)
'english navy seizing island on way north' is the wordplay.
'english' becomes 'e' (abbreviation).
'navy' becomes 'RN' (abbreviation for Royal Navy).
'seizing' means one lot of letters goes inside another.
'island' becomes 'elba' (Italian island).
'on way north' says the letters should be written backwards (in a down clue, letters go up or 'north').
'rn' placed around 'elba' is 'relban'.
'relban' backwards is 'nabler'.
'e'+'nabler'='ENABLER'
(Other definitions for enabler that I've seen before include "One who makes possible" , "Someone who makes something possible for another" , "One giving authority" , "I give authority" , "It makes (thing) possible" .)