Crossword Genius

Defence group dine outside at start of day (5)

Ross

I believe the answer is:

sunup

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'start of day' is the definition.
The answer and definition can be both related to time as well as being singular nouns.
Maybe they are linked in a way I don't understand?

'defence group dine outside' is the wordplay.
'defence group' becomes 'nu' (I can't justify this - if you can you should give a lot more credence to this answer).
'dine' becomes 'sup' (synonyms).
'outside' indicates putting letters inside (some letters go outside others).
'nu' put inside 'sup' is 'SUNUP'.

'at' is the link.

Can you help me to learn more?

(Other definitions for sunup that I've seen before include "Daybreak (in America)" , "First appearance of star" , "Solar appearance (US)" , "early in America" , "Literal version of dawn" .)

I've seen this clue in The Daily Express.
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