Arrogant person out of bed before dawn (7)
I believe the answer is:
upstart
'arrogant person' is the definition.
'out of bed before dawn' is the wordplay.
'out of bed' becomes 'up' (synonyms).
'before' says to put letters next to each other.
'dawn' becomes 'start' (I've seen this in other clues).
'up'+'start'='UPSTART'
(Other definitions for upstart that I've seen before include "Person recently elevated and arrogant with it" , "Arrogant and presumptuous person" , "Recent promotees, say, behaving with arrogance" , "Tap rust (anag.)" , "mushroom" .)