Some thought it a nice vessel once (7)
I believe the answer is:
titanic
'once' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I can't see?
'some thought it a nice vessel' is the wordplay.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'TITANIC' can be found hidden inside 'thought it a nice vessel'.
Can you help me to learn more?
(Other definitions for titanic that I've seen before include "Ship that sank in 1912" , "Giant" , "Nineteen twelve iceberg victim" , "Of enormous size, like the stricken liner" , "Film success (1997) - ship disaster (1912)" .)