Game of golf needing fortieth round (5)
I believe the answer is:
rugby
'game' is the definition.
(rugby is a kind of game)
'golf needing fortieth round' is the wordplay.
'golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
'needing fortieth' becomes 'ruby' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'round' means one lot of letters goes inside another.
'g' inserted inside 'ruby' is 'RUGBY'.
'of' acts as a link.
(Other definitions for rugby that I've seen before include "Tom Brown's school" , "Famous public school" , "Game; town" , "Team sport" , "Public school - sport" .)