After Dominic Cork had dismissed David Fulton, Dean, a left-armer, claimed the scalps of Ed Smith, Carl Hooper and Nigel Llong. Mark Ealham and Matthew Fleming then dug in to deny Derbyshire until bad light forced an early close with Kent on 104 for 5, a lead of nine.
At Worcester, the home side, following on 254 behind Yorkshire's 455 for 9 declared, were soon in trouble after Darren Gough and Gavin Hamilton removed the openers Elliott Wilson and Philip Weston in 15 balls as Worcestershire closed at 19 for 2, still needing 235 to stave off an innings defeat.
Nottinghamshire all-rounder Paul Franks provided the only resistance as England Under-19s were bowled out for 139 by their Pakistan counterparts at Chester-le-Street. Franks contributed 31 from 34 balls but received little support as England chased Pakistan's 245 for 6.
Bad weather wiped out the day's play at Old Trafford, where Leicestershire were unable to add to their 218 for 7 against title-chasing Lancashire.