Slow boat merely capsized … (3)
I believe the answer is:
tub
'slow boat' is the definition.
(I've seen this before)
'merely capsized' is the wordplay.
'merely' becomes 'but' ('but' can mean 'only' or 'merely').
'capsized' says the letters should be written in reverse.
'but' back-to-front is 'TUB'.
(Other definitions for tub that I've seen before include "Small round container" , "Wide bucket-like vessel" , "Large open container" , "Large plant pot" , "Bath; clumsy ship" .)