Buck cast up reed (4)
I believe the answer is:
deer
'buck cast' is the definition.
'deer' can be an answer for 'buck' (thesaurus). I am not sure about the 'cast' bit.
'up reed' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'reed' backwards is 'DEER'.
Can you help me to learn more?
(Other definitions for deer that I've seen before include "Ruminant(s) -- breed of 13 down" , "maybe does" , "The male is a stag" , "Ruminant mammal with antlers and hooves" , "Antler-bearing animal" .)