Brash girl's behind advert regularly featured (7)
I believe the answer is:
ladette
'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 understand how this works, but
'advert' could be 'ad' ('ad' can be a synonym of 'advert') and 'ad' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for ladette that I've seen before include "boozy girl" , "Rowdy youngster" , "Girl likely to drink lots" , "She knocks back too much" , "Foul-mouthed, beer-swilling girl" .)