Not exactly eighteen holes! (6)
I believe the answer is:
around
'not' is the definition.
Both the definition and answer are adverbs. Maybe you can see a link between them that I can't see?
'exactly eighteen holes' is the wordplay.
'exactly' becomes 'a' (I can't explain this - if you can you should give a lot more credence to this answer).
'eighteen holes' becomes 'round' (a round of golf includes eighteen holes).
'a'+'round'='AROUND'
Can you help me to learn more?
(Other definitions for around that I've seen before include "Close to" , "Present; approximately" , "About, here and there in circular way?" , "On all sides or near" , "Somewhere in the vicinity" .)