Sleepwear's close to neckwear (7)
I believe the answer is:
nightie
'sleepwear's' is the definition.
(I've seen this before)
'close to neckwear' is the wordplay.
'close to' becomes 'nigh'.
'neckwear' becomes 'tie' (tie is a kind of neckwear).
'nigh'+'tie'='NIGHTIE'
(Other definitions for nightie that I've seen before include "Sleeping garment" , "garment worn in bed" , "one put on after dark" , "Bedtime wear" , "Does evening wear" .)