However choppy neither vessel leaves island (12)
I believe the answer is:
nevertheless
'however' is the definition.
(synonyms)
'choppy neither vessel leaves island' is the wordplay.
'choppy' indicates an anagram.
'leaves' indicates named letters should be taken away.
'island' becomes 'i'.
'neither'+'vessel'='neithervessel'
'neithervessel' with 'i' taken out is 'nethervessel'.
'nethervessel' is an anagram of 'NEVERTHELESS'.
(Other definitions for nevertheless that I've seen before include "at any rate" , "In spite of everything" , "But always" , "anyhow" , "Still" .)