It wags at a young lady! (7)
I believe the answer is:
flapper
'young lady' is the definition.
(flapper is a kind of young lady)
'it wags at a' is the wordplay.
'it wags' becomes 'flap' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'at' says to put letters next to each other.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'flap'+'per'='FLAPPER'
Can you help me to learn more?
(Other definitions for flapper that I've seen before include "20s fashionable woman" , "Fashionable young woman of the 1920s" , "Unconventional nineteen twenties girl" , "fashionable female in twenties" , "one inclined to panic" .)