Astounded by stupid son on lorry (10)
I believe the answer is:
dumbstruck
'astounded' is the definition.
(thesaurus)
'stupid son on lorry' is the wordplay.
'stupid' becomes 'dumb' ('dumb' can be similar in meaning to 'stupid'**).
'son' becomes 's'.
'on' is a charade indicator (letters next to each other).
'lorry' becomes 'truck' (I've seen this before).
'dumb'+'s'+'truck'='DUMBSTRUCK'
'by' acts as a link.
(Other definitions for dumbstruck that I've seen before include "Temporarily deprived of speech by shock" , "Gobsmacked" , "Speechless with amazement" .)