Ultimately golf, not cricket, is a source of entertainment (7)
I believe the answer is:
funfair
'source of entertainment' is the definition.
'funfair' can be an answer for 'entertainment' (I've seen this before). I am not sure about the 'source of' bit.
'ultimately golf not cricket' is the wordplay.
'ultimately' says to take the final letters.
'not' becomes 'un' (I've seen this in other clues).
'cricket' becomes 'fair' (I can't justify this - if you can you should believe this answer much more).
The last letter of 'golf' is 'f'.
'f'+'un'+'fair'='FUNFAIR'
'is a' is the link.
Can you help me to learn more?
(Other definitions for funfair that I've seen before include "entertainment for the family?" , "Swings and roundabouts" , "Amusement park" , "Amusements here" , "place of amusement" .)