Day in Ascot rearranged for groups of eight (6)

I believe the answer is:

octads

'groups of eight' is the definition.
(I've seen this before)

'day in ascot rearranged' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'in' means one lot of letters goes inside another.
'rearranged' is an anagram indicator.
'ascot' anagrammed gives 'octas'.
'd' inserted into 'octas' is 'OCTADS'.

'for' acts as a link.

(Another definition for octads that I've seen is " Groups of eight".)

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