Strange tale about river warning (5)
I believe the answer is:
alert
'warning' is the definition.
(I know that alert is a type of warning)
'strange tale about river' is the wordplay.
'strange' indicates an anagram.
'about' is an insertion indicator.
'river' becomes 'r' (abbreviation).
'tale' with letters rearranged gives 'alet'.
'alet' placed around 'r' is 'ALERT'.
(Other definitions for alert that I've seen before include "alarm" , "Put on guard, warn" , "Lively" , "Warning signal" , "Watchful, sharp" .)