Blimey - spread tons on top of highway (7)
I believe the answer is:
strewth
'blimey' is the definition.
(both are expressions of surprise)
'spread tons on top of highway' is the wordplay.
'spread' becomes 'strew' (to strew is to spread or scatter around).
'tons' becomes 't'.
'on top of' suggests taking the first letters.
The initial letter of 'highway' is 'h'.
'strew'+'t'+'h'='STREWTH'
(Other definitions for strewth that I've seen before include "Fancy" , "blimey" , "Mild expletive" , "Antipodean exclamation" , "mild oath" .)