Small portion -- this writer eats it (4)
I believe the answer is:
mite
'small portion' is the definition.
(a mite is a tiny amount of something)
'this writer eats it' is the wordplay.
'this writer' becomes 'me' (the person who is writing these clues).
'eats' means one lot of letters goes inside another.
'me' going around 'it' is 'MITE'.
(Other definitions for mite that I've seen before include "Microscopic creature" , "The Biblical widow's offering" , "Very small creature or child" , "Microscopic animal" , "Tiny arachnid, person or amount of money" .)