Thin top erected over Aussie truck (6)
I believe the answer is:
dilute
'thin' is the definition.
(synonyms)
'top erected over aussie truck' is the wordplay.
'top' becomes 'lid' (lid is a kind of top).
'erected' says the letters should be written in reverse (in a down clue, letters go up).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'aussie truck' becomes 'ute' (Australian abbreviation for utility vehicle).
'lid' written backwards gives 'dil'.
'dil'+'ute'='DILUTE'
(Other definitions for dilute that I've seen before include "Reduce concentration of a liquid" , "lost concentration" , "Add water to another liquid" , "Thin down by adding water" , "having less concentration" .)