The egg the female is protecting is one of a pair (4)
I believe the answer is:
shoe
'one of a pair' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'the egg the female is protecting' is the wordplay.
'the egg' becomes 'o' (an 'o' resembles an egg).
'the female' becomes 'she' ('she' refers to a woman).
'is protecting' indicates putting letters inside (inserted letters are held or protected).
'o' placed into 'she' is 'SHOE'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for shoe that I've seen before include "Slip-on or lace-up?" , "mule, perhaps?" , "Playing card box" , "Maybe court" , "Eg, brogue" .)