The writer ultimately diminished great singer as tragic high-flier (6)
I believe the answer is:
icarus
'tragic high-flier' is the definition.
'icarus' can be an answer for 'high-flier' (I've seen this before). I'm not sure about the 'tragic' bit.
'the writer ultimately diminished great singer' is the wordplay.
'the writer' becomes 'I' (the person who is writing these clues).
'ultimately diminished great singer' becomes 'carus' (I can't explain this - if you can you should give a lot more credence to this answer).
'i'+'carus'='ICARUS'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for icarus that I've seen before include "Ill-fated son of Daedalus" , "he tried flying" , "His false wings melted" , "In Greek myth, he flew tooclose to the sun" , "Son of Daedalus whose wings melted fatally" .)