Old submarine docks at eastern island — almost everyone knows it (4,6)
I believe the answer is:
open secret
'everyone knows it' is the definition.
(something everybody secretly knows)
'old submarine docks at eastern island almost' is the wordplay.
'old submarine docks at' becomes 'opens' (I can't explain this - if you can you should believe this answer much more).
'eastern' becomes 'e'.
'island' becomes 'crete' (Greek island).
'almost' means to remove the last letter (most of the word but not all of it).
'crete' with its last letter taken away is 'cret'.
'opens'+'e'+'cret'='OPEN SECRET'
(Other definitions for open secret that I've seen before include "common knowledge" , "Everyone knows it" , "Centre pose (anag.)" , "Generally known matter" , "Private matter actually known to many" .)