Spoil ruler with means of identification (7)
I believe the answer is:
marking
'means of identification' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'spoil ruler' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'ruler' becomes 'king' (king is a kind of ruler).
'mar'+'king'='MARKING'
'with' is the link.
Can you help me to learn more?
(Other definitions for marking that I've seen before include "Rating (school work)" , "Labelling" , "Staying close to an opposing player" , "assessment" , "Assessing script" .)