Big lorry, one originally carrying paintings on top (5)
I believe the answer is:
artic
'big lorry' is the definition.
(short for articulated lorry)
'one originally carrying paintings on top' is the wordplay.
'one' becomes 'i' (roman numeral).
'originally' indicates taking the first letters.
'paintings' becomes 'art' (I've seen this before).
'on top' says to put letters next to each other (down clues only).
The first letter of 'carrying' is 'c'.
'i'+'c'='ic'
'ic' put after 'art' is 'ARTIC'.
(Other definitions for artic that I've seen before include "rig" , "Lorry (abbrev.)" , "Heavy transporter" , "Large lorry" , "Polar region" .)