Egg within is by a large opening (7)
I believe the answer is:
initial
'opening' is the definition.
(I've seen this before)
'egg within is by a large' is the wordplay.
'egg' becomes 'nit' (a nit is an insect egg).
'within' means one lot of letters goes inside another.
'is by' becomes 'ii' (more than one I. I am not sure about the 'by' bit.).
'large' becomes 'L' (eg in clothes sizes).
'nit' put into 'ii' is 'initi'.
'initi'+'a'+'l'='INITIAL'
Can you help me to learn more?
(Other definitions for initial that I've seen before include "Original; beginning" , "Name's first letter" , "First letter of a name" , "Introductory" , "Relating to the beginning" .)