King on board galley, drifting in the main (7)
I believe the answer is:
largely
'in the main' is the definition.
(thesaurus)
'king on board galley drifting' is the wordplay.
'king' becomes 'r' (r is abbreviation for rex).
'on board' is an insertion indicator.
'drifting' is an anagram indicator.
'galley' anagrammed gives 'lagely'.
'r' inserted into 'lagely' is 'LARGELY'.
(Other definitions for largely that I've seen before include "Generally" , "To a great extent" , "Allergy (anag.)" , "Principally" , "as a rule" .)