Captains perhaps in privy at sea (5)
I believe the answer is:
heads
'captains perhaps' is the definition.
(captaining is a kind of heading)
'privy at sea' is the wordplay.
'privy' becomes 'head' (head is a kind of privy).
'at sea' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'head'+'s'='HEADS'
'in' is the link.
Can you help me to learn more?
(Other definitions for heads that I've seen before include "Noddles; nuts" , "Ship's latrine; addicts" , "Leaders" , "Obverse result of coin-tossing" , "Chiefs" .)