Hoots from river, one owl primarily hollowed-out trees (5)
I believe the answer is:
riots
'hoots' is the definition.
(both can mean a fun thing or person)
'river one owl primarily hollowed-out trees' is the wordplay.
'river' becomes 'ri' (I am not sure about this - if you are sure you should believe this answer much more).
'primarily' says to take the initial letters.
'hollowed out' says to hollow out the word (remove centre letters).
'trees' with its centre removed is 'ts'.
The initial letter of 'owl' is 'o'.
'ri'+'o'+'ts'='RIOTS'
'from' acts as a link.
(Other definitions for riots that I've seen before include "Violent disturbances" , "Wild disturbances by crowds" , "Protests" , "Acts of violence by unruly mobs" , "Public disturbances" .)