Wild animal captured by Magnus (3)
I believe the answer is:
gnu
'wild animal' is the definition.
(I know that gnu is a type of wild animal)
'captured by magnus' is the wordplay.
'captured by' indicates the answer is hidden within the clue.
'GNU' can be found hidden inside 'magnus'.
(Other definitions for gnu that I've seen before include "Wildebeest" , "Literally wild beast" , "Large heavy antelope" , "Gun for a kind of deer" , "Strange gun for an antelope" .)