Always the first woman to go by river (4)
I believe the answer is:
'always' is the definition.
'the first woman to go by river' is the wordplay.
'the first woman' becomes 'Eve' (first woman in the bible).
'to go by' says to put letters next to each other.
'river' becomes 'r'.
(Other definitions for ever that I've seen before include "To the end of time" , "On any occasion" , "Always, at any time" , "Possibly" , "Continuously" .)