First light (4)
'first light?' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'first light?' is the wordplay.
'first' becomes 'one' (I've seen this in another clue).
'light?' becomes 'across' (I am not sure about this - if you are sure you should believe this answer much more).
'one'+'across'='ONE ACROSS'
Can you help me to learn more?
(Other definitions for one across that I've seen before include "being absent from the puzzle" , "Usually the first clue (though not here!)" .)
'first light' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for sunup that I've seen before include "Solar rising" , "Solar appearance (US)" , "Dawn (mainly US)" , "Literal version of dawn" , "Daybreak (in America)" .)
'first light' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for dawn that I've seen before include "Sunrise" , "Light in East" , "Break of day" , "Aurora was its goddess" , "beginning" .)