Darren Bent's first goal for Derby sets up victory against a spirited Chesterfield side in a gripping FA Cup fourth round tie.
Bent, on loan from Aston Villa, opened the scoring for Steve McClaren's Championship promotion chasers with a close-range first-half header.
But the League One visitors dominated possession and wasted several good chances after that. Will Hughes secured Derby's fifth round place with a late solo goal.
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