Spot oversight, arresting knight in seedy club (9)
'spot' is the definition.
'oversight arresting knight in seedy club' is the wordplay.
'oversight' becomes 'care' (I can't explain this - if you can you should give a lot more credence to this answer).
'arresting' means one lot of letters goes inside another.
'knight' becomes 'n' (chess).
'in' means one lot of letters goes inside another.
'seedy' is an anagram indicator (I've seen this in other clues).
'club' anagrammed gives 'bucl'.
'n' put into 'bucl' is 'buncl'.
'care' going around 'buncl' is 'CARBUNCLE'.
Can you help me to learn more?
(Other definitions for carbuncle that I've seen before include "Sort of boil; a gem", "National Gallery's Sainsbury Wing?", "Skin eruption - gemstone", "Severe abscess in the skin", "Architectural eyesore".)