Dawn's daughter certainly not heavy (8)
I believe the answer is:
daylight
'dawn's' is the definition.
(I've seen this before)
'daughter certainly not heavy' is the wordplay.
'daughter' becomes 'd' (genealogical abbreviation).
'certainly' becomes 'ay' (variant spelling of 'aye').
'not heavy' becomes 'light' ('light' is the opposite to 'heavy').
'd'+'ay'+'light'='DAYLIGHT'
(Other definitions for daylight that I've seen before include "Clear distance from" , "When the sun's up (time for robbery?)" , "Clear space" , "Clear difference between rivals" , "robbery, sort of" .)