Trio trapping quiet river shark (8)
I believe the answer is:
thresher
'shark' is the definition.
(I know that thresher is a type of shark)
'trio trapping quiet river' is the wordplay.
'trio' becomes 'three' (synonyms).
'trapping' indicates putting letters inside.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'river' becomes 'r' (abbreviation for river).
'three' enclosing 'sh' is 'threshe'.
'threshe'+'r'='THRESHER'
(Other definitions for thresher that I've seen before include "Combine" , "mimic's relative?" , "Flail" , "Person or machine that cuts grass, wheat, etc" , "Subject to a trampling" .)