Game of golf interrupting fortieth wedding (5,5)
I believe the answer is:
rugby union
'game' is the definition.
(I've seen this before)
'golf interrupting fortieth wedding' is the wordplay.
'golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
'interrupting' is an insertion indicator.
'fortieth' becomes 'ruby' (a ruby anniversary marks 40 years).
'wedding' becomes 'union' (I've seen this before).
'g' inserted within 'ruby' is 'rugby'.
'rugby'+'union'='RUGBY UNION'
'of' acts as a link.
(Other definitions for rugby union that I've seen before include "Fifteen-a-side sport - Olympic event in 1900, 1908, 1920 and 1924" , "activity for hooligans?" , "Sort of football" , "Fifteen player sport ..." , "Fifteen-a-side game" .)