Almost everything eaten by sea creature's foul! (7)
I believe the answer is:
squalid
'foul' is the definition.
(both can mean disgusting)
'almost everything eaten by sea creature's' is the wordplay.
'almost' means to remove the last letter (almost the whole word is kept).
'everything' becomes 'all' (all things).
'eaten by' indicates putting letters inside (inserted letters get eaten).
'sea creature' becomes 'squid' (squids live in the sea).
'all' with its final letter taken away is 'al'.
'al' inserted into 'squid' is 'SQUALID'.
(Other definitions for squalid that I've seen before include "Foul" , "Run-down - filthy" , "Seedy" , "run down" , "Extremely dirty" .)