Buried under storage container is a Japanese poem (5)
I believe the answer is:
tanka
'japanese poem' is the definition.
'tanka' can be an answer for 'poem' (I've seen this before). I am not certain of the 'japanese' bit.
'buried under storage container is a' is the wordplay.
'buried under storage container' becomes 'tank' (I am not sure about this - if you are sure you should believe this answer much more).
'tank'+'a'='TANKA'
Can you help me to learn more?
(Other definitions for tanka that I've seen before include "some Japanese verse" , "Type of Japanese poem" .)