Worsening classes during last month ultimately rally (10)
I believe the answer is:
degeneracy
'worsening' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'classes during last month ultimately rally' is the wordplay.
'classes' becomes 'genera' (more than one genus - a class or category).
'during' means one lot of letters goes inside another.
'last month' becomes 'dec' (short for December, last month of the year).
'ultimately' says to take the final letters.
The last letter of 'rally' is 'y'.
'genera' put into 'dec' is 'degenerac'.
'degenerac'+'y'='DEGENERACY'
Can you help me to learn more?
(Other definitions for degeneracy that I've seen before include "Physical and moral decline" , "State of moral decline" , "corruption" , "Immoral behaviour" .)