Crossword Genius

Giants substitute seen after blunder in field (5)

Ross

I believe the answer is:

ogres

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'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" .)

I've seen this clue in The Sun.
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