Slow time in last day (6)

I believe the answer is:

retard

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'slow' is the definition.
(I know that slow can be written as retard)

'time in last day' is the wordplay.
'time' becomes 'ret' (I've seen this in other clues).
'in last day' becomes 'ard' (I can't justify this - if you can you should give a lot more credence to this answer).
'ret'+'ard'='RETARD'

Can you help me to learn more?

(Other definitions for retard that I've seen before include "Impede like trader" , "Detain" , "Hold back -- trader (anag)" , "Darted about to hold back or delay" , "Hold back, slow progress" .)

I've seen this clue in The Telegraph.

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