Seals it somehow to put to sea (3,4)
I believe the answer is:
set sail
'put to sea' is the definition.
(I've seen this in another clue)
'seals it somehow' is the wordplay.
'somehow' indicates an anagram.
'seals'+'it'='sealsit'
'sealsit' is an anagram of 'SET SAIL'.
'to' is the link.
(Other definitions for set sail that I've seen before include "Leave for (sea) voyage" , "Embark on a voyage by ship" , "Left port" , "have started journey" , "Begin a voyage" .)