Blue thing old boys sees starting, even ends in a metropolis (9)
I believe the answer is:
obscenity
'blue thing' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'old boys sees starting even ends in a metropolis' is the wordplay.
'starting' indicates taking the first letters.
'ends' says to hollow out the word (remove centre letters).
'in' means one lot of letters goes inside another.
'a metropolis' becomes 'city' (a metropolis is a large city).
'even' with its centre taken out is 'en'.
The first letters of 'old boys sees' is 'obs'.
'en' placed inside 'city' is 'cenity'.
'obs'+'cenity'='OBSCENITY'
Can you help me to learn more?
(Other definitions for obscenity that I've seen before include "indecency" , "Strong language" , "pornography?" , "Smut" , "Something offensive or disgusting" .)