Thin cover put around pick-up truck (6)
I believe the answer is:
dilute
'thin' is the definition.
('dilute' can be a synonym of 'thin')
'cover put around pick-up truck' is the wordplay.
'cover' becomes 'lid' (lid is a kind of cover).
'put around' says the letters should be written in reverse.
'pick-up truck' becomes 'ute' (Australian abbreviation for utility vehicle).
'lid' reversed gives 'dil'.
'dil'+'ute'='DILUTE'
(Other definitions for dilute that I've seen before include "Adulterate" , "Lessen the strength of a solution" , "Make (a liquid) thinner" , "Add liquid to lessen strength" , "The time when something ceases to have validity" .)