Busy person duly disheartened, old chap’s upset (6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

D [ul] Y (dis-heartened = inner letters removed), then O (abbreviation for old) + MAN (chap) all reversed (upset = upwards in a down clue). Dynamo = slang for an energetic person who runs around doing everything.