The day before is always short (3)
I believe the answer is:
eve
'the day before' is the definition.
(I've seen this before)
'always short' is the wordplay.
'always' becomes 'ever' ('ever' can be a synonym of 'always').
'short' means to remove the last letter.
'ever' with its final letter removed is 'EVE'.
'is' acts as a link.
(Other definitions for eve that I've seen before include "Day before a festival" , "The night before a festival" , "Woman in a happy place" , "Adam & - -" , "Adam's wife" .)