Hard black varnish containers - one not closed (5)
I believe the answer is:
japan
'hard black varnish' is the definition.
(I've seen this in another clue)
'containers one not closed' is the wordplay.
'containers' becomes 'jap' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one' becomes 'ane' (synonyms).
'not closed' means to remove the last letter.
'ane' with its final letter taken away is 'an'.
'jap'+'an'='JAPAN'
Can you help me to learn more?
(Other definitions for japan that I've seen before include "It's a string of more than 3,000 islands" , "Glossy, black varnish; country" , "East Asian country" , "Far Eastern island country" , "Country in East Asia" .)