Trouble after groups go to sea (3,4)
I believe the answer is:
set sail
'go to sea' is the definition.
(I've seen this in another clue)
'trouble after groups' is the wordplay.
'trouble' becomes 'ail' (synonyms).
'after' means one lot of letters go next to another.
'groups' becomes 'sets' (a set is a group of things).
'ail' after 'sets' is 'SET SAIL'.
(Other definitions for set sail that I've seen before include "left land behind" , "Left port" , "Embark on a voyage by ship" , "Leave port (by ship)" , "Begin a voyage" .)