I ate crab at sea, hence my indigestion? (8)
I believe the answer is:
bacteria
'hence my indigestion?' is the definition.
(indigestion may be caused by bacteria)
'i ate crab at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'i'+'ate'+'crab'='iatecrab'
'iatecrab' with letters rearranged gives 'BACTERIA'.
(Other definitions for bacteria that I've seen before include "Germs or bugs" , "Infectious microorganisms" , "Microscopic organisms, good and bad" , "Micro-organisms, often pathogens" , "Tiny organisms found in bait race" .)