Astonished as lorry roars ahead (13)
I believe the answer is:
thunderstruck
'astonished' is the definition.
(synonyms)
'lorry roars ahead' is the wordplay.
'lorry' becomes 'truck' (I've seen this before).
'roars' becomes 'thunders' ('thunder' can be a synonym of 'roar').
'ahead' is a charade indicator (letters next to each other) (some letters go in front of others).
'truck' put after 'thunders' is 'THUNDERSTRUCK'.
'as' is the link.
(Other definitions for thunderstruck that I've seen before include "Dumbfounded" , "Extremely surprised" , "Stupefied" , "Amazed" , "Very surprised, shocked" .)