Waxer and Waner are demonstrably cheeky! (4)
I believe the answer is:
moon
'waner are demonstrably cheeky' is the definition.
I can't tell whether this definition defines the answer.
'waxer and' is the wordplay.
'waxer' becomes 'moo' (I am not sure about this - if you are sure you should believe this answer much more).
'and' becomes 'n' (common abbreviation for 'and').
'moo'+'n'='MOON'
Can you help me to learn more?
(Other definitions for moon that I've seen before include "Phoebe" , "Natural satellite of a planet" , "who headed a religious 29 [CULT]" , "Appear listless" , "drummer?" .)