Lowest point in Eridanus, to the west (5)
I believe the answer is:
nadir
'lowest point' is the definition.
'in eridanus to the west' is the wordplay.
'in' indicates the answer is hidden within the clue.
'to the west' shows that the letters should be reversed in order.
'ridan' can be found hidden inside 'eridanus'.
'ridan' in reverse letter order is 'NADIR'.
(Other definitions for nadir that I've seen before include "Very bottom, farthest from zenith" , "Drain into the lowest point (5)" , "It's the depths" , "Point of greatest despair" , "Dinar (anag.)" .)