Captains fish in empty seas (8)
I believe the answer is:
skippers
'captains' is the definition.
('skipper' can be a synonym of 'captain')
'fish in empty seas' is the wordplay.
'fish' becomes 'kipper' (type of fish).
'in' means one lot of letters goes inside another.
'empty seas' becomes 'ss' (I can't explain this - if you can you should believe this answer much more).
'kipper' placed into 'ss' is 'SKIPPERS'.
Can you help me to learn more?
(Other definitions for skippers that I've seen before include "taking PE?" , "Masters" , "Sea captains" , "Informal ship captains" , "Captains of ships or sports teams" .)