Slowing down, a month before Lee wasted allowance (12)
I believe the answer is:
deceleration
'slowing down' is the definition.
(the opposite of acceleration)
'a month before lee wasted allowance' is the wordplay.
'a month' becomes 'dec' (abbreviation for December).
'before' is a charade indicator (letters next to each other).
'wasted' is an anagram indicator.
'allowance' becomes 'ration' (I've seen this before).
'lee' is an anagram of 'ele'.
'dec'+'ele'+'ration'='DECELERATION'
(Other definitions for deceleration that I've seen before include "Process of slowing down" , "Slowing of velocity" .)