Material from river reportedly turning up negative (5)
I believe the answer is:
nylon
'material' is the definition.
(nylon is a kind of material)
'river reportedly turning up negative' is the wordplay.
'river reportedly' becomes 'lyn' (I am not sure about this - if you are sure you should believe this answer much more).
'turning' is a reversal indicator.
'up' is a reversal indicator (in down clue: letters go upwards).
'negative' becomes 'no' (no is a kind of negative).
'lyn' reversed gives 'nyl'.
'no' in reverse letter order is 'on'.
'nyl'+'on'='NYLON'
'from' is the link.
Can you help me to learn more?
(Other definitions for nylon that I've seen before include "Hosiery material" , "Man-made fibre" , "A thermoplastic" , "may be damaged by running" , "where ladder can appear?" .)