Indolent, dull, lacking energy initially (4)

I believe the answer is:

idle

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'indolent' is the definition.
(similar in meaning)

'dull lacking energy initially' is the wordplay.
'dull' becomes 'id' (I can't justify this - if you can you should give a lot more credence to this answer).
'initially' suggests taking the first letters.
The first letters of 'lacking energy' is 'le'.
'id'+'le'='IDLE'

Can you help me to learn more?

(Other definitions for idle that I've seen before include "Frivolous" , "Not doing much" , "out of a job" , "Lied about doing nothing" , "Useless" .)

I've seen this clue in The Telegraph.

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