Seeing new son's birth (7)
I believe the answer is:
genesis
'birth' is the definition.
(I've seen this before)
'seeing new son's' is the wordplay.
'new' indicates anagramming the letters.
'seeing' with letters rearranged gives 'genesi'.
'genesi'+'s'='GENESIS'
(Other definitions for genesis that I've seen before include "Birth, beginnings" , "OT opening" , "Origin (of the Old Testament?)" , "name of group" , "Creation" .)