River extremely strange? Not so (5)
I believe the answer is:
false
'not so' is the definition.
(I've seen this in another clue)
'river extremely strange?' is the wordplay.
'river' becomes 'fal' (Cornish river).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'strange' with its centre taken out is 'se'.
'fal'+'se'='FALSE'
(Other definitions for false that I've seen before include "Artificial" , "Not genuine or real" , "Misleading" , "Not true" , "Spurious, untrue" .)