River rodent in credulous story (9)
I believe the answer is:
narrative
'story' is the definition.
(synonyms)
'river rodent in credulous' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'rodent' becomes 'rat' (rat is a kind of rodent).
'in' is an insertion indicator.
'credulous' becomes 'naive' (similar in meaning).
'r'+'rat'='rrat'
'rrat' inserted within 'naive' is 'NARRATIVE'.
(Other definitions for narrative that I've seen before include "Story line" , "Account of connected events, a story" , "Report" , "Verbal account" , "Telling a story" .)