Upset last month over mail being misdirected, it's asserted (7)
I believe the answer is:
claimed
'asserted' is the definition.
(claiming is a kind of asserting)
'upset last month over mail being misdirected' is the wordplay.
'upset' is a reversal indicator.
'last month' becomes 'dec' (short for December, last month of the year).
'over' indicates putting letters inside.
'being misdirected' indicates an anagram (I've seen this in other clues).
'dec' backwards is 'ced'.
'mail' is an anagram of 'laim'.
'ced' going around 'laim' is 'CLAIMED'.
'it's' acts as a link.
(Other definitions for claimed that I've seen before include "Declared" , "Asserted one's right to" , "Alleged" , "Called for" , "Asserted right to or interest in" .)