Giant blunder on field concerning (4)
I believe the answer is:
ogre
'giant' is the definition.
(I know that ogre is a mythical humanoid race)
'blunder on field concerning' is the wordplay.
'blunder on field' becomes 'og' (own goal eg in football).
'concerning' becomes 're' (both can mean on a particular topic).
'og'+'re'='OGRE'
(Other definitions for ogre that I've seen before include "Has a monstrous role" , "terrible person" , "Ugly giant" , "The giant is some goer" , "Fierce giant" .)