Dog really captured fiend (4)
I believe the answer is:
ogre
'fiend' is the definition.
(synonyms)
'dog really captured' is the wordplay.
'captured' indicates the answer is hidden within the clue.
'OGRE' is hidden in the letters of 'dog really'.
(Other definitions for ogre that I've seen before include "Grim(m) beast?" , "Terrifying monster" , "terrible person" , "Man-eating monster of fairy tales" , "bad-tempered person" .)