Song serious about golf (5)
I believe the answer is:
dirge
'song' is the definition.
(dirge is a kind of song)
'serious about golf' is the wordplay.
'serious' becomes 'dire' (I've seen this in another clue).
'about' indicates putting letters inside.
'golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
'dire' going around 'g' is 'DIRGE'.
(Other definitions for dirge that I've seen before include "Keen; lament" , "Sombre music" , "Melancholy song" , "lamentable composition" , "Mournful chant" .)