Trimmed unnatural river structure (4)
I believe the answer is:
weir
'river structure' is the definition.
(type of dam)
'trimmed unnatural' is the wordplay.
'trimmed' means to remove the last letter (the final letter is trimmed off).
'unnatural' becomes 'weird' (weird can mean supernatural or magical).
'weird' with its final letter taken off is 'WEIR'.
(Other definitions for weir that I've seen before include "Barrier" , "feature of some rivers" , "Something that may be seen across river" , "type of block" , "hazard during 14" .)