Most foolish fibs about river sediment? On the contrary (8)
I believe the answer is:
silliest
'most foolish' is the definition.
('silly' can be similar in meaning to 'foolish')
'fibs about river sediment? on the contrary' is the wordplay.
I cannot really see how this works, but
'fibs' could be 'lies' (informal term for lies) and 'lies' is present in the answer.
'sediment?' could be 'silt' (I've seen this in another clue) and 'silt' is found in the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for silliest that I've seen before include "It's Ellis (anag)" , "Most ridiculous" , "Most inane" , "Most unwise" , "Most foolish" .)