Identical line contributed to ruler showing star quality (9)
I believe the answer is:
twinkling
'star quality' is the definition.
Both the answer and definition are singular nouns.
Maybe there's a link between them I don't understand?
'identical line contributed to ruler showing' is the wordplay.
'identical' becomes 'twin' (I've seen this in another clue).
'line' becomes 'l' (used when specifying particular lines from a poem).
'contributed to' means one lot of letters goes inside another.
'ruler showing' becomes 'king' (I am not sure about this - if you are sure you should believe this answer much more).
'l' put inside 'king' is 'kling'.
'twin'+'kling'='TWINKLING'
Can you help me to learn more?
(Other definitions for twinkling that I've seen before include "Second" , "Jiffy" , "Sparkling - moment" , "Sparkling like a star" , "Instant" .)