Rat swallows a river insect (6)
I believe the answer is:
scarab
'insect' is the definition.
(I know that Scarab is a type of insect)
'rat swallows a river' is the wordplay.
'rat' becomes 'scab' (synonyms).
'swallows' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'a'+'r'='ar'
'scab' enclosing 'ar' is 'SCARAB'.
(Other definitions for scarab that I've seen before include "Sacred Egyptian beetle" , "Holy beetle" , "Egyptian stone" , "crawler" , "Sacred dung-beetle" .)