One on course has this useful sounding headgear (8)
I believe the answer is:
handicap
'one on course has this' is the definition.
(golf course)
'useful sounding headgear' is the wordplay.
'useful' becomes 'handy' (I've seen this before).
'sounding' shows a homophone (sound like).
'headgear' becomes 'cap' (cap is a kind of headgear).
'handy' sounds like 'handi'.
'handi'+'cap'='HANDICAP'
(Other definitions for handicap that I've seen before include "It impedes or hinders" , "Impede" , "race" , "Disadvantage to offset an advantage to equalise chances of winning" , "Grand National for example" .)