Ruin shoe on a golf course, finally (8)
I believe the answer is:
sabotage
'ruin' is the definition.
(thesaurus)
'shoe on a golf course finally' is the wordplay.
'shoe' becomes 'sabot' (I've seen this in another clue).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
'finally' indicates one should take the final letters.
The final letter of 'course' is 'e'.
'sabot'+'a'+'g'+'e'='SABOTAGE'
(Other definitions for sabotage that I've seen before include "Vandalism" , "spoil things" , "Disrupt" , "Scupper" , "Deliberate and malicious destruction" .)