Nell, evidently fast mistress (7)
I believe the answer is:
quickly
'nell evidently' is the definition.
I can't judge whether this definition defines the answer.
'fast mistress' is the wordplay.
'fast' becomes 'quick' (similar in meaning).
'mistress' becomes 'ly' (I am not sure about this - if you are sure you should believe this answer much more).
'quick'+'ly'='QUICKLY'
Can you help me to learn more?
(Other definitions for quickly that I've seen before include "Speedily" , "Rapidly; promptly" , "Mistress of Eastcheap tavern" , "At speed" .)