Sea still not quite stormy? Be on your way (3,4)
I believe the answer is:
set sail
'be on your way' is the definition.
(both can mean to begin a journey)
'sea still not quite stormy?' is the wordplay.
'not quite' means to remove the last letter.
'stormy?' is an anagram indicator (I've seen 'stormy' mean this).
'sea'+'still'='seastill'
'seastill' with its final letter taken off is 'seastil'.
'seastil' with letters rearranged gives 'SET SAIL'.
(Other definitions for set sail that I've seen before include "have started journey" , "Begin a voyage" , "Leave for (sea) voyage" , "Left port" , "Leave port (by ship)" .)