Eighteen, for a golf course; why not indulging at the nineteenth? (8)
I believe the answer is:
teetotal
'nineteenth?' is the definition.
Both the answer and definition are adjectives. Perhaps you can see a link between them that I can't see?
'eighteen for a golf course why not indulging at the' is the wordplay.
I cannot quite understand how this works, but
'for' could be 'to' (eg both can mean 'in the opinion of') and 'to' is present in the answer.
'a' is present in the answer.
'course' could be 'e' and 'e' is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for teetotal that I've seen before include "Pledged to abstain from alcohol" , "Non-drinking" , "drunk? No way" , "Never drinking alcohol" , "Avoiding alcohol" .)