Passing a month with effortlessness (7)
I believe the answer is:
decease
'passing' is the definition.
(thesaurus)
'a month with effortlessness' is the wordplay.
'a month' becomes 'dec' (abbreviation for December).
'with' says to put letters next to each other.
'effortlessness' becomes 'ease' (both can mean a lack of difficulty).
'dec'+'ease'='DECEASE'
(Other definitions for decease that I've seen before include "Death, die" , "end of life" .)