Ready: line up boxes (6)
I believe the answer is:
crates
'boxes' is the definition.
(crate is a kind of box)
'ready line up' is the wordplay.
'ready' becomes 'set' ('set' can be similar in meaning to 'ready').
'line' becomes 'arc' (arc is a kind of line**).
'up' is a reversal indicator (in down clue: letters go upwards).
'set'+'arc'='setarc'
'setarc' back-to-front is 'CRATES'.
(Other definitions for crates that I've seen before include "Jalopies" , "Boxes for shipping goods" , "Packing cases" , "Rough wooden boxes" .)