Sun the first sign of the dawn (5)
I believe the answer is:
start
'dawn' is the definition.
(I've seen this in other clues)
'sun the first sign of the' is the wordplay.
'sun' becomes 'star' (I've seen this before).
'the first sign of' suggests taking the first letters.
The first letter of 'the' is 't'.
'star'+'t'='START'
(Other definitions for start that I've seen before include "Take the first step" , "Jump suddenly" , "Set in motion" , "Begin - jump" , "Begin with sudden movement of surprise" .)