Decomposes in a short month, say, in a way (6)
I believe the answer is:
decays
'decomposes' is the definition.
(I've seen this before)
'short month say in a way' is the wordplay.
'short month' becomes 'Dec' (abbreviation for December; 'short' asks for a short form).
'in a way' is an anagram indicator.
'say' is an anagram of 'ays'.
'dec'+'ays'='DECAYS'
'in a' acts as a link.
(Other definitions for decays that I've seen before include "Rots, moulders away" , "Rots, disintegrates" , "Rots and decomposes" , "Decomposes or falls into ruin" , "Goes off" .)