Animal raised, in theory (3)
I believe the answer is:
roe
'animal' is the definition.
(I've seen this before)
'raised in theory' is the wordplay.
'raised' shows that the letters should be reversed in order.
'in' indicates a hidden word.
'eor' is hidden within 'theory'.
'eor' written backwards gives 'ROE'.
(Other definitions for roe that I've seen before include "shy animal" , "A mass of fish-eggs" , "Caviar" , "Swift runner" , "Part of fish, sounds like go through water" .)