I ate crabs endlessly at sea -- they can cause illness (8)
I believe the answer is:
bacteria
'they can cause illness' is the definition.
'i ate crabs endlessly at sea' is the wordplay.
'endlessly' means to remove the last letter.
'at sea' indicates anagramming the letters.
'i'+'ate'+'crabs'='iatecrabs'
'iatecrabs' with its final letter removed is 'iatecrab'.
'iatecrab' anagrammed gives 'BACTERIA'.
(Other definitions for bacteria that I've seen before include "Single-cell organisms, can be good or bad for you" , "Possible cause of disease" , "Cabaret I found containing microscopic organisms" , "I eat crab (anag)" , "Causes of many diseases" .)