Last month, F Prefect’s friend becoming corrupt! (8)
I believe the answer is:
decadent
'corrupt' is the definition.
(I've seen this before)
'last month f prefect's friend' is the wordplay.
'last month' becomes 'dec' (December, last month of the year).
'f prefect's friend' becomes 'adent' (I can't explain this - if you can you should give a lot more credence to this answer).
'dec'+'adent'='DECADENT'
'becoming' is the link.
Can you help me to learn more?
(Other definitions for decadent that I've seen before include "Luxuriously self-indulgent" , "Morally deficient; declining" , "Decanted in corrupt way" , "abandoned" , "Self-indulgent or immoral" .)