Giant blunder in defence concerning (4)
I believe the answer is:
ogre
'giant' is the definition.
(I know that ogre is a type of giant)
'blunder in defence concerning' is the wordplay.
'blunder in defence' becomes 'OG' (own goal).
'concerning' becomes 're' (both can mean on a particular topic).
'og'+'re'='OGRE'
(Other definitions for ogre that I've seen before include "Cruel, fearsome person" , "Has a monstrous role" , "Bugbear" , "Terrifying person" , "Grim(m) beast?" .)