Sailor has huge cake outside (5)
I believe the answer is:
bosun
'sailor' is the definition.
(I know that bosun is a type of Jack)
'huge cake outside' is the wordplay.
'huge' becomes 'os' (abbreviation for outsize).
'cake' becomes 'bun' (bun is a type of cake).
'outside' is an insertion indicator (some letters go outside others).
'os' put into 'bun' is 'BOSUN'.
'has' acts as a link.
(Other definitions for bosun that I've seen before include "Crew's foreman" , "A dimunitive boatswain" , "An abbreviated boatswain" , "Bonus for a small boatswain" , "A short boatswain" .)