A month on, cavity's decaying (8)
I believe the answer is:
decrepit
'decaying' is the definition.
(thesaurus)
'a month on cavity's' is the wordplay.
'a month' becomes 'Dec' (abbreviation for December).
'on' becomes 're' ('on' can be mean 'with reference to').
'cavity' becomes 'pit' (synonyms).
'dec'+'re'+'pit'='DECREPIT'
(Other definitions for decrepit that I've seen before include "In the last stages of decay" , "Run-down by old age or hard use" , "Cited rep for being old and dilapidated" , "ruined by neglect" , "past its best" .)