Late night party dress extremely pretty, Chanel's never without one (7)
I believe the answer is:
pyjamas
'late night party dress' is the definition.
(as in a pyjama party)
'extremely pretty chanel's never without one' is the wordplay.
'extremely pretty' becomes 'py' (I can't explain this - if you can you should give a lot more credence to this answer).
'chanel's never' becomes 'jamais' ('never' in French - Coco Chanel was French).
'without' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'jamais' with 'i' taken away is 'jamas'.
'py'+'jamas'='PYJAMAS'
Can you help me to learn more?
(Other definitions for pyjamas that I've seen before include "Bed clothes" , "Night clothes" , "Nightclothes" , "Night attire" , "Pays jam poorly to wear them" .)