Creatures go to river, sick with arsenic (8)
I believe the answer is:
gorillas
'creatures' is the definition.
(gorillas are animals)
'go to river sick with arsenic' is the wordplay.
'to river' becomes 'r' (abbreviation for river).
'sick' becomes 'ill' (synonyms).
'with' is a charade indicator (letters next to each other).
'arsenic' becomes 'As' (As is the chemical symbol for arsenic).
'go'+'r'+'ill'+'as'='GORILLAS'
(Other definitions for gorillas that I've seen before include "Large powerful apes from Africa" , "Largest primates" .)