Cynical as an egg (10)

'cynical' is the definition.
The answer and definition are different parts of speech. However, past participle verbs and adjectives can occasionally define each other.

'an egg' is the wordplay.
I cannot really understand how this works, but
'an' could be 'i' (Roman numeral for one) and 'i' is present in the answer.
'egg' could be 'o' (an 'o' resembles an egg) and 'o' is found within the answer.
This may be the basis of the clue (or it may be nonsense).

'as' acts as a link.

Can you help me to learn more?