Come by in boat at sea (6)
I believe the answer is:
obtain
'come by' is the definition.
(thesaurus)
'in boat at sea' is the wordplay.
'at sea' indicates an anagram.
'in'+'boat'='inboat'
'inboat' is an anagram of 'OBTAIN'.
(Other definitions for obtain that I've seen before include "Get or receive" , "Get one's hands on" , "Get - secure" , "Gain possession of" , "Acquire, secure" .)