Saying it's a mix you beat up and put an egg in (5)
I believe the answer is:
axiom
'saying' is the definition.
(axiom is a kind of saying)
'a mix you beat up and put an egg in' is the wordplay.
I cannot quite understand how this works, but
'a' is within the answer.
'an' could be 'i' (Roman numeral for one) and 'i' is found in the answer.
'egg' could be 'o' (an 'o' resembles an egg) and 'o' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'it's' is the link.
Can you help me to learn more?
(Other definitions for axiom that I've seen before include "Universally accepted proposition" , "Self-evident principle" , "Truth assumed to be self-evident" , "Statement regarded as self-evidently true" , "Obviously true statement" .)