Puss! Puss! (7)
I believe the answer is:
copycat
'puss puss' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'puss puss' is the wordplay.
'puss' becomes 'copy' (I am not sure about this - if you are sure you should believe this answer much more).
'puss' becomes 'cat' (I've seen this in other clues).
'copy'+'cat'='COPYCAT'
Can you help me to learn more?
(Other definitions for copycat that I've seen before include "One who imitates another" , "not the original one" , "Slavish imitator" , "Emulator" , "Person imitating others" .)