Policy behind last month's worsening performance (7)
I believe the answer is:
decline
'worsening performance' is the definition.
'decline' can be an answer for 'worsening' (thesaurus). I am unsure of the 'performance' bit.
'policy behind last month's' is the wordplay.
'policy' becomes 'line' (the party line is a party's policy on something).
'behind' is a charade indicator (letters next to each other).
'last month' becomes 'dec' (short for December, last month of the year).
'line' after 'dec' is 'DECLINE'.
Can you help me to learn more?
(Other definitions for decline that I've seen before include "Refuse - dip" , "Refuse to accept" , "go through cases" , "Evelyn Waugh novel" , "Start to fail" .)