Some theory about eggs (3)
I believe the answer is:
roe
'eggs' is the definition.
(fish eggs)
'some theory about' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'about' says the letters should be written backwards.
'eor' is hidden in the letters of 'theory'.
'eor' backwards is 'ROE'.
(Other definitions for roe that I've seen before include "Lobster's ripe ovary" , "eg Caviare" , "If hard, eggs" , "Swift runner" , "Food from fish" .)