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

I believe the answer is:

dynamo

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'busy person' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'duly disheartened old chap's upset' is the wordplay.
'disheartened' means to remove the middle letters (the middle or heart is removed).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'chap' becomes 'man' (chap can mean a man).
'upset' shows that the letters should be reversed in order.
'duly' with its middle removed is 'dy'.
'o'+'man'='oman'
'oman' back-to-front is 'namo'.
'dy'+'namo'='DYNAMO'

Can you help me to learn more?

(Other definitions for dynamo that I've seen before include "Monday (anag)" , "Type of electrical generator" , "Forceful person" , "Source of electric current" , "Electricity generator or energetic person" .)

I've seen this clue in The Independent.

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