Attorney, unknown, on holiday with Dawn (8)
I believe the answer is:
daybreak
'dawn' is the definition.
(I know that dawn can be written as daybreak)
'attorney unknown on holiday' is the wordplay.
'attorney' becomes 'D.A.' (abbreviation for District Attorney).
'unknown' becomes 'y' (algebra).
'on' says to put letters next to each other.
'holiday' becomes 'break' (break can mean a holiday or time off).
'da'+'y'+'break'='DAYBREAK'
'with' is the link.
(Other definitions for daybreak that I've seen before include "Cockcrow" , "Dawn" , "When light first appears" , "Sunrise" , "very short holiday" .)