Large container merely overturned (3)
I believe the answer is:
tub
'large container' is the definition.
'tub' can be an answer for 'container' (tub is a kind of container). I am unsure of the 'large' bit.
'merely overturned' is the wordplay.
'merely' becomes 'but' ('but' can mean 'only' or 'merely').
'overturned' says the letters should be written backwards.
'but' written backwards gives 'TUB'.
Can you help me to learn more?
(Other definitions for tub that I've seen before include "Bath; ice cream container" , "Large plant pot" , "Barrel; boat" , "'Broad, open vessel (3)'" , "Maybe it sailed" .)