Garment shortened - after dark that is (7)

I believe the answer is:

nightie

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'garment shortened after dark that is' is the definition.
'nightie' can be an answer for 'garment' (I've seen this before). I am unsure of the rest of the definition.

'dark that is' is the wordplay.
'dark' becomes 'night' (it's dark at nighttime).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'night'+'ie'='NIGHTIE'

Can you help me to learn more?

(Other definitions for nightie that I've seen before include "Clothing worn in bed" , "Skimpy attire" , "Bed attire (colloq.)" , "Bedtime habit" , "What retiring lady assumes" .)

I've seen this clue in The Globe and Mail.

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