Four score two fours by end of day (6)
I believe the answer is:
eighty
'four score' is the definition.
(I've seen this before)
'two fours by end of day' is the wordplay.
'two fours' becomes 'eight' (I've seen this before).
'by' is a charade indicator (letters next to each other).
'end of' says to take the final letters.
The final letter of 'day' is 'y'.
'eight'+'y'='EIGHTY'
(Other definitions for eighty that I've seen before include "XXC" , "LXXX" , "score less than a hundred" , "22 [FOUR] scores" , "'Nineteen -- Four' (novel)" .)