The day before is not entirely smooth (3)
I believe the answer is:
eve
'the day before' is the definition.
(the eve of something is the night before)
'not entirely smooth' is the wordplay.
'not entirely' means to remove the last letter (most but not all of the word).
'smooth' becomes 'even' (similar in meaning).
'even' with its last letter removed is 'EVE'.
'is' acts as a link.
(Other definitions for eve that I've seen before include "comes before Jan. 1st" , "Day before (an event)" , "Adam's wife" , "See 27" , "Lady of the previous night" .)