From last month, editor rejoiced (7)
I believe the answer is:
exulted
'rejoiced' is the definition.
(exulting is a kind of rejoicing)
'from last month editor' is the wordplay.
'from' becomes 'ex' ('from' in Latin).
'last month' becomes 'ult' (short for 'last month' in correspondence).
'editor' becomes 'ed' (abbreviation).
'ex'+'ult'+'ed'='EXULTED'
(Other definitions for exulted that I've seen before include "crew" , "Was jubilant" , "Rejoiced exceedingly" , "Felt elation" , "Rejoiced greatly as in triumph" .)