Fish thrash, rising both sides of flowing creek (8)
I believe the answer is:
mackerel
'fish' is the definition.
(I know this)
'thrash rising both sides of flowing creek' is the wordplay.
'thrash' becomes 'lam' (both can mean to hit).
'rising' is a reversal indicator (going up in a down clue).
'both sides of' indicates putting letters inside (one word stretches round both sides of another).
'flowing' is an anagram indicator.
'lam' back-to-front is 'mal'.
'creek' with letters rearranged gives 'ckere'.
'mal' going around 'ckere' is 'MACKEREL'.
(Other definitions for mackerel that I've seen before include "Food fish related to tuna" , "Food fish with a blue-green back" , "Common food fish" , "Marine food-fish" , "Small stripped fish, very tasty" .)