Box on the right shows sign of damage (6)
I believe the answer is:
crater
'sign of damage' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'box on the right' is the wordplay.
'box' becomes 'crate' (crate is a kind of box).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'the right' becomes 'r' (common abbreviation).
'crate'+'r'='CRATER'
'shows' acts as a link.
Can you help me to learn more?
(Other definitions for crater that I've seen before include "Volcanic cavity" , "Geological depression" , "Cavity formed by explosion" , "Volcanic depresssion" , "Volcano's mouth" .)