Sluggishness of one on illegal drug in train, tripping (7)

I believe the answer is:

inertia

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'sluggishness' is the definition.
(I've seen this in another clue)

'one on illegal drug in train tripping' is the wordplay.
'one' becomes 'I' (Roman numeral).
'on' means one lot of letters go next to another.
'illegal drug' becomes 'e' (E is another name for ecstasy. I am not sure about the 'illegal' bit.).
'in' is an insertion indicator.
'tripping' indicates anagramming the letters.
'train' anagrammed gives 'nrtia'.
'e' put into 'nrtia' is 'nertia'.
'i'+'nertia'='INERTIA'

'of' acts as a link.

Can you help me to learn more?

(Other definitions for inertia that I've seen before include "Admonish" , "I tear in (anag.)" , "'Sluggishness, indolence (7)'" , "Lack of activity" , "Sluggishness - sloth" .)

I've seen this clue in The Times.

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