Fiend therefore went up .. (4)
I believe the answer is:
ogre
'fiend' is the definition.
(I know that fiend can be written as ogre)
'therefore went up' is the wordplay.
'therefore' becomes 'ergo' (I've seen this before).
'went up' says the letters should be written in reverse (I've seen 'going up' mean this).
'ergo' written backwards gives 'OGRE'.
(Other definitions for ogre that I've seen before include "Wicked and inhuman person" , "Jack's giant perhaps" , "One to gobble flesh" , "Wicked giant" , "Brute" .)