Sleepwear (7)
'sleepwear' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nightie that I've seen before include "cover for retiring" , "Sleepwear" , "Sleeping garment" , "Light sleeping gown" , "Woman in bed in this" .)
(Other definitions for pyjamas that I've seen before include "Sleeping suit" , "Pays jam poorly to wear them" , "Sleeping-suit" , "Sleeping attire" , "Jam a spy into these at night" .)