Vacant seat unknown by fabulous river (4)
I believe the answer is:
styx
'fabulous river' is the definition.
'styx' can be an answer for 'river' (mythical river). I'm not certain of the 'fabulous' bit.
'vacant seat unknown by' is the wordplay.
'vacant' suggests removing the centre.
'unknown' becomes 'y' (algebra).
'by' becomes 'x' (multiplication sign**).
'seat' with its centre removed is 'st'.
'st'+'y'+'x'='STYX'
Can you help me to learn more?
(Other definitions for styx that I've seen before include "The last river one crosses" , "River surrounding Hades" , "'River of Hate' in Hades" , "Mythological river ferried by Charon" , "One of the underworld rivers" .)