Otter that regularly eats river fish (5)
I believe the answer is:
tetra
'fish' is the definition.
(tetra is a kind of fish)
'otter that regularly eats river' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'eats' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'otter'+'that'='otterthat'
The alternate letters of 'otterthat' are 'teta'.
'teta' enclosing 'r' is 'TETRA'.
(Other definitions for tetra that I've seen before include "With four parts" , "in groups of four" , "Small tropical fish" , "Swimmer" , "Wrongful act (Law)" .)