Brash girl's behind advert regularly featured (7)
I believe the answer is:
Here is my best explanation:
'brash girl's behind' is the definition.
The definition suggests a singular noun which matches the answer.
'advert regularly featured' is the wordplay.
I cannot quite see how this works, but
'advert' could be 'ad' ('ad' can be a synonym of 'advert') and 'ad' is found within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for ladette that I've seen before include "binge-drinker", "Uncouth youth drinking lager in a short skirt?", "Coarse young woman", "boisterous madam", "She knocks back too much", "Tomboy", "Young woman with a loutish lifestyle?", "Crude young woman", "lively girl" and "Loutish girl".)