Masses of round containers outside back of house (6)
I believe the answer is:
oceans
'masses' is the definition.
(ocean can mean a mass or large amount of something)
'of round containers outside back of house' is the wordplay.
'of' becomes 'o''.
'round containers' becomes 'cans' (can is a kind of container. I am not sure about the 'round' bit.).
'outside' is an insertion indicator.
'back of' suggests the final letters.
The last letter of 'house' is 'e'.
'cans' placed around 'e' is 'ceans'.
'o'+'ceans'='OCEANS'
Can you help me to learn more?
(Other definitions for oceans that I've seen before include "big seas" , "Large seas" , "Vast bodies of water" , "Strange canoes on these waters" , "Bodies of salt water" .)