Slyness of the salt at sea (7)
I believe the answer is:
stealth
'slyness' is the definition.
(thesaurus)
'the salt at sea' is the wordplay.
'at sea' is an anagram indicator.
'the'+'salt'='thesalt'
'thesalt' is an anagram of 'STEALTH'.
'of' is the link.
(Other definitions for stealth that I've seen before include "quality needed by cat burglars" , "Skill in furtive activity" , "underhand dealing" , "Halt set in furtive manner" , "By which taxes go up" .)