Thin cover lifted on truck (6)
I believe the answer is:
dilute
'thin' is the definition.
(I know that thin can be written as dilute)
'cover lifted on truck' is the wordplay.
'cover' becomes 'lid' (lid is a kind of cover).
'lifted' shows that the letters should be reversed in order (going up in down clue).
'on' means one lot of letters go next to another.
'truck' becomes 'ute' (Australian abbreviation for utility vehicle).
'lid' backwards is 'dil'.
'dil'+'ute'='DILUTE'
(Other definitions for dilute that I've seen before include "Make (a liquid) thinner" , "Reduce concentration of a liquid" , "Weaken by adding water" , "The time when something ceases to have validity" , "Adulterate" .)