Seafood soup rotten at end of month (7)
I believe the answer is:
octopus
'seafood' is the definition.
(I know that octopus is a type of seafood)
'soup rotten at end of month' is the wordplay.
'rotten' is an anagram indicator.
'at end of' is a charade indicator (letters next to each other) (some letters go at the end of others).
'month' becomes 'oct' (abbreviation for October).
'soup' is an anagram of 'opus'.
'opus' after 'oct' is 'OCTOPUS'.
(Other definitions for octopus that I've seen before include "Ocean-dweller" , "pernicious organisation" , "Long-tentacled creature" , "Mollusc with long tentacles" , "Eight-armed cephalopod" .)