Yellow River associated with unknown disease (10)
I believe the answer is:
chickenpox
'disease' is the definition.
(chickenpox is a kind of disease)
'yellow river associated with unknown' is the wordplay.
'yellow' becomes 'chicken' (as in cowardly).
'river' becomes 'po' (Italian river).
'associated with' means one lot of letters go next to another.
'unknown' becomes 'x' (algebra).
'chicken'+'po'+'x'='CHICKENPOX'
(Other definitions for chickenpox that I've seen before include "Disease" , "Youngsters spotted with it" .)