Crossword Genius

Really good 40-day period on beat (9)

Ross

I believe the answer is:

excellent

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'beat' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

'really good 40-day period on' is the wordplay.
'really good e0 day' becomes 'excel' (I can't justify this - if you can you should believe this answer much more).
'period' becomes 'lent' (Lent is an example).
'on' means one lot of letters go next to another.
'excel'+'lent'='EXCELLENT'

Can you help me to learn more?

(Other definitions for excellent that I've seen before include "capital" , "Great!" , "Quite splendid, extremely good" , "'Superior, first class (9)'" , "A-One" .)

I've seen this clue in The Sun.
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