Ship's gaol, and who should be in it! (7)
I believe the answer is:
brigand
'who should be in it' is the definition.
I can't tell whether this definition defines the answer.
'ship's gaol' is the wordplay.
'ship' becomes 'brig' (I've seen this before).
'gaol' becomes 'and' (I am not sure about this - if you are sure you should believe this answer much more).
'brig'+'and'='BRIGAND'
'and' is the link.
Can you help me to learn more?
(Other definitions for brigand that I've seen before include "Highway robber" , "desperado" , "Plunderer" , "Pirate" , "Robber in wild places" .)