Night before a big day (3)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
eve
'night before a big day' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eve that I've seen before include "Brink" , "Lady first appearing" , "One from Genesis" , "Adam & - -" , "Day before a festival" .)