Sins of old age were his: terrible sins hidden by king of France (7)
I believe the answer is:
rossini
'sins of old age were his' is the definition.
The definition suggests a singular noun which matches the answer.
'terrible sins hidden by king of france' is the wordplay.
'terrible' is an anagram indicator.
'hidden by' is an insertion indicator.
'king of france' becomes 'roi' (I can't explain this - if you can you should give a lot more credence to this answer).
'sins' anagrammed gives 'ssin'.
'ssin' put within 'roi' is 'ROSSINI'.
Can you help me to learn more?
(Other definitions for rossini that I've seen before include "He composed operas" , "Italian opera composer" , "Composer of The Thieving Magpie" , "Great musician" , "Italian scorer" .)