Farm animal that is eating river shellfish (6)
I believe the answer is:
cowrie
'shellfish' is the definition.
(I know that cowrie is a type of gastropod)
'farm animal that is eating river' is the wordplay.
'farm animal' becomes 'cow' (type of farm animal).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'eating' is an insertion indicator.
'river' becomes 'r' (abbreviation for river).
'cow'+'ie'='cowie'
'cowie' placed around 'r' is 'COWRIE'.
(Other definitions for cowrie that I've seen before include "Shell sometimes used as form of money" , "Marine gastropod with a shiny shell once used as money in some parts" , "Shell once used as currency" , "Brightly marked seashell" , "Mollusc (shell used as money)" .)