Copycat is over neat (5)
I believe the answer is:
xerox
'copycat is over neat' is the definition.
I can't judge whether this defines the answer.
'copycat is over neat' is the wordplay.
'copycat' becomes 'rex' (I can't explain this - if you can you should give a lot more credence to this answer).
'is over' says the letters should be written in reverse.
'neat' becomes 'ox' (neat historically meant a cow or ox).
'rex' backwards is 'xer'.
'xer'+'ox'='XEROX'
Can you help me to learn more?
(Other definitions for xerox that I've seen before include "Trade mark photocopier" , "Copying machine" , "Copy machine" , "Photocopy" .)