Dab oar at sea on a boat (6)
I believe the answer is:
aboard
'on a boat' is the definition.
(I've seen this before)
'dab oar at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'dab'+'oar'='daboar'
'daboar' with letters rearranged gives 'ABOARD'.
(Other definitions for aboard that I've seen before include "On an aircraft" , "... [CRUISE SHIP] here, perhaps" , "on a liner, say" , "at sea, say?" , "Alongside" .)