Go back again to evil giant (4)
I believe the answer is:
ogre
'evil giant' is the definition.
(I've seen this before)
'go back again' is the wordplay.
'back' says the letters should be written in reverse.
'again' becomes 're' (prefix meaning 'again').
'go' back-to-front is 'og'.
'og'+'re'='OGRE'
'to' acts as a link.
(Other definitions for ogre that I've seen before include "ugly one" , "Bugbear" , "Terrifying giant" , "Monstrous creature" , "Has a monstrous role" .)