Almost last, group of musicians overturn deadline (9)
I believe the answer is:
ultimatum
'deadline' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'almost last group of musicians overturn' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'last' becomes 'ultimate' (similar in meaning).
'group of musicians overturn' becomes 'um' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ultimate' with its final letter taken away is 'ultimat'.
'ultimat'+'um'='ULTIMATUM'
Can you help me to learn more?
(Other definitions for ultimatum that I've seen before include "Final demand or statement of terms" , "Threat" , "Final warning" , "Uncompromising demand" , "Final point" .)