Rotten month, I see (6)

I believe the answer is:
septic
'rotten' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?
'month i see' is the wordplay.
'month' becomes 'sept' (Sept is an example).
'see' becomes 'C' (the word for the letter, according to Chambers).
'sept'+'i'+'c'='SEPTIC'
Can you help me to learn more?
(Other definitions for septic that I've seen before include "Type of tank" , "Contaminated with disease-causing bacteria" , "Putrid" , "Of wound, filled with pus" , "Of wound, forming pus" .)
