Astound in front, sweetheart (4,4)
I believe the answer is:
bowl over
'astound' is the definition.
(thesaurus)
'front sweetheart' is the wordplay.
'front' becomes 'bow' (bow is a kind of front).
'sweetheart' becomes 'lover' (sweetheart can mean a lover).
'bow'+'lover'='BOWL OVER'
'in' acts as a link.
(Other definitions for bowl over that I've seen before include "Thoroughly delight" , "Overwhelm with surprise or emotion - at cricket?" , "Slay" , "Greatly surprise" , "wow" .)