Each April, at sea, leave for islands (11)
I believe the answer is:
archipelago
'islands' is the definition.
(I've seen this before)
'each april at sea leave' is the wordplay.
'at sea' indicates an anagram.
'leave' becomes 'go' (to go is to depart or leave).
'each'+'april'='eachapril'
'eachapril' anagrammed gives 'archipela'.
'archipela'+'go'='ARCHIPELAGO'
'for' is the link.
(Other definitions for archipelago that I've seen before include "Large island group" , "Fiji, perhaps" , "Extensive group of islands" , "watery domain" , "the Aegean, originally" .)