Croesus keeps some deer (3)
I believe the answer is:
roe
'deer' is the definition.
(roe deer is a kind of deer)
'croesus keeps some' is the wordplay.
'some' says the answer is hidden in the clue.
'ROE' is hidden within 'croesus keeps'.
(Other definitions for roe that I've seen before include "Mass of fish eggs" , "If hard, eggs" , "Food from fish" , "Lobster's ripe ovary" , "shy animal" .)