Metal dug up in Arno River (4)
I believe the answer is:
iron
'metal dug' is the definition.
'iron' can be an answer for 'metal' (thesaurus). I am unsure of the 'dug' bit.
'up in arno river' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'in' indicates a hidden word.
'nori' is hidden in the letters of 'arno river'.
'nori' backwards is 'IRON'.
Can you help me to learn more?
(Other definitions for iron that I've seen before include "medicinal preparation for the anaemic?" , "Box-shaped instrument of torture -- no meridian (anag)" , "Remove creases from" , "Metal obtained from haematite" , "Club; element" .)