Reason for one to eat egg (6)
I believe the answer is:
sanity
'reason' is the definition.
(I've seen this before)
'for one to eat egg' is the wordplay.
'for one' becomes 'say' (both can mean 'for example').
'to eat' is an insertion indicator.
'egg' becomes 'nit' (a nit is an insect egg).
'say' placed around 'nit' is 'SANITY'.
(Other definitions for sanity that I've seen before include "Rational condition" , "Healthy mental state" , "Sound powers of mind" , "Rationality" , "Sound sense" .)