Set about, before daybreak (5)
I believe the answer is:
ready
'set' is the definition.
('ready' can be a synonym of 'set')
'about before daybreak' is the wordplay.
'about' becomes 're' (regarding**).
'before' is a charade indicator (letters next to each other).
'daybreak' becomes 'ady' (ady is day anagrammed 'broken').
're'+'ady'='READY'
(Other definitions for ready that I've seen before include "for immediate use" , "Poised for action" , "Available now" , "Willing (to do something), prepare" , "Waiting" .)