Writer had vacated library in vain (4)
I believe the answer is:
idly
'in vain' is the definition.
(I've seen this before)
'writer had vacated library' is the wordplay.
'writer' becomes 'i' (the person who is writing these clues).
'had' becomes 'd' (I've seen this in other clues).
'vacated' means to remove the middle letters.
'library' with its centre taken out is 'ly'.
'i'+'d'+'ly'='IDLY'
(Other definitions for idly that I've seen before include "So lacking purpose" , "In a lazy manner" , "How people may stand by" , "Indolently" , "Lazily, without due attention" .)