Underground river, or possibly one arch (7)
I believe the answer is:
acheron
'underground river' is the definition.
'acheron' can be an answer for 'river' (I've seen this before). I am unsure of the 'underground' bit.
'possibly one arch' is the wordplay.
'possibly' indicates anagramming the letters.
'one'+'arch'='onearch'
'onearch' anagrammed gives 'ACHERON'.
'or' is the link.
Can you help me to learn more?
(Other definitions for acheron that I've seen before include "one classical route to underworld" , "A river of Hades" .)