Habit into which one might slip when retiring? (9)
I believe the answer is:
nightgown
'habit into which one might slip when retiring?' is the definition.
(clothing worn when retiring to bed)
This is all the clue.
(Other definitions for nightgown that I've seen before include "evening wear" , "Garment worn in bed" , "Sleeping garment" , "Cover for retirement" , "Clothing" .)