Last month, leaders undermined Lisbon Treaty (3)
I believe the answer is:
ult
'last month' is the definition.
(short for 'last month' in correspondence)
'leaders undermined lisbon treaty' is the wordplay.
'leaders' says to take the initial letters.
The initial letters of 'undermined lisbon treaty' is 'ULT'.
(Other definitions for ult that I've seen before include "In brief, notation for last month" , "Of last month, in a letter" , "Abbreviation in correspondence for last month" , "Notation in business correspondence meaning last month" , "Last month (abbr)" .)