One Aussie truck reversed in case (4)
I believe the answer is:
etui
'in case' is the definition.
(I've seen this before)
'one aussie truck reversed' is the wordplay.
'one' becomes 'i' (Roman numeral).
'aussie truck' becomes 'ute' (Australian abbreviation for utility vehicle).
'reversed' shows that the letters should be reversed in order.
'i'+'ute'='iute'
'iute' reversed gives 'ETUI'.
(Other definitions for etui that I've seen before include "Small case for small articles" , "Small sewing bag" , "Small case for small things" , "Ornamental ladies' bag for small articles" , "Needlework case" .)