Horrible cries in river creating worry (8)
I believe the answer is:
exercise
'worry' is the definition.
(I've seen this before)
'horrible cries in river' is the wordplay.
'horrible' indicates an anagram.
'in' means one lot of letters goes inside another.
'river' becomes 'exe' (River Exe).
'cries' anagrammed gives 'ercis'.
'ercis' put within 'exe' is 'EXERCISE'.
'creating' is the link.
(Other definitions for exercise that I've seen before include "Task performed to develop skill" , "Keep-fit" , "Practice session" , "Practice piece of work" , "problem" .)