The night before (3)
'the night before' is the definition.
(eve can mean the night before something)
This is the entire clue.
(Other definitions for eve that I've seen before include "Female" , "Night before an event" , "Previous day" , "First mate" , "She reportedly lived first" .)
(Other definitions for twas that I've seen before include "Contraction of it was" , "It used to be, formerly" .)