Fools back dull American swimmer (8)
I believe the answer is:
stingray
'swimmer' is the definition.
(type of fish)
'fools back dull american' is the wordplay.
'fools' becomes 'nits' (I've seen this before).
'back' says the letters should be written backwards.
'dull american' becomes 'gray' (I can't justify this - if you can you should believe this answer much more).
'nits' in reverse letter order is 'stin'.
'stin'+'gray'='STINGRAY'
Can you help me to learn more?
(Other definitions for stingray that I've seen before include "Marine creature with venomous tail" , "Dangerous fish" , "Poisonous spined fish" , "Large venomous fish -- it's angry (anag)" , "Fish with venomous tail" .)