Oarsman or his headwear (6)
I believe the answer is:
boater
'oarsman' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for boater that I've seen before include "Flat-topped straw hat" , "Hat; eg, rower" , "sailor, perhaps" , "Sailor's straw hat" , "Stiff hat made of straw" .)