Copper with cook, appalling nest-stealer (6)
I believe the answer is:
cuckoo
'nest-stealer' is the definition.
I can't judge whether this definition defines the answer.
'copper with cook appalling' is the wordplay.
'copper' becomes 'Cu' (chemical symbol).
'with' is a charade indicator (letters next to each other).
'appalling' indicates an anagram (an appalling spelling of the letters).
'cook' anagrammed gives 'ckoo'.
'cu'+'ckoo'='CUCKOO'
Can you help me to learn more?
(Other definitions for cuckoo that I've seen before include "One calling on the hour" , "Bird (with a bad reputation)" , "sound? Hardly!" , "fool" , "Intruder bird in the nest" .)