Well, at least she isn’t mousy! (3)
I believe the answer is:
cat
'least she isn't mousy' is the definition.
I can't judge whether this definition defines the answer.
'well at' is the wordplay.
'well' becomes 'c' (I can't justify this - if you can you should believe this answer much more).
'c'+'at'='CAT'
Can you help me to learn more?
(Other definitions for cat that I've seen before include "Animal - one with nine tails!" , "Animal (domesticated or wild)" , "one's pyjamas? They're an excellent thing" , "Pussy" , "Malicious gossip" .)