Shellfish might be a different story if ingesting source of effluent (6)
I believe the answer is:
oyster
'shellfish' is the definition.
(oyster is a kind of shellfish)
'a different story if ingesting source of effluent' is the wordplay.
'a different' indicates an anagram.
'if ingesting' means one lot of letters goes inside another.
'source of' says to take the initial letters.
The initial letter of 'effluent' is 'e'.
'story' anagrammed gives 'oystr'.
'oystr' placed around 'e' is 'OYSTER'.
'might be' acts as a link.
(Other definitions for oyster that I've seen before include "Means to cross London" , "Pearl-producing shellfish" , "person unwilling to communicate" , "Maybe grey" , "aphrodisiac?" .)