Wavering, likely to survive crossing a river (8)
I believe the answer is:
variable
'wavering' is the definition.
(thesaurus)
'likely to survive crossing a river' is the wordplay.
'likely to survive' becomes 'viable' (I've seen this in another clue).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'river' becomes 'r' (abbreviation for river).
'a'+'r'='ar'
'viable' going around 'ar' is 'VARIABLE'.
(Other definitions for variable that I've seen before include "Not steady or regular" , "Unsteady" , "mathematician's expression" , "Prone to change" , "can be changed" .)