Amaze a son with this developing (8)
I believe the answer is:
astonish
'amaze' is the definition.
('astonish' can be a synonym of 'amaze')
'a son with this developing' is the wordplay.
'with' says to put letters next to each other.
'developing' indicates an anagram.
'son' after 'this' is 'thisson'.
'thisson' with letters rearranged gives 'stonish'.
'a'+'stonish'='ASTONISH'
(Other definitions for astonish that I've seen before include "Dumbfound" , "Startle" , "Shock with surprise" , "Provoke wonder" , "Flabbergast" .)