Really good 40-day period on beat (9)
I believe the answer is:
excellent
'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" .)