River creature caught twice eating or on its back (4)
I believe the answer is:
croc
'river creature' is the definition.
(short for crocodile)
'caught twice eating or on its back' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'twice' shows that letters should be duplicated.
'eating' indicates putting letters inside.
'on its back' says the letters should be written in reverse (flipped backwards).
'or' reversed gives 'ro'.
'c' doubled is 'cc'.
'cc' enclosing 'ro' is 'CROC'.
(Other definitions for croc that I've seen before include "Small reptile" , "Dangerous reptile" , "Wild animal" , "Small animal" , "that might be sitting on your foot" .)