Giants substitute seen after blunder in field (5)
I believe the answer is:
ogres
'giants' is the definition.
(I know this)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ogres that I've seen before include "'Monsters, fiends (5)'" , "Fairytale giants" , "Giants in fairytales" , "Gorse (anag.)" , "Formidable people" .)