Colossal vessel contains ointment (5)
I believe the answer is:
salve
'ointment' is the definition.
(I know that ointment can be written as salve)
'colossal vessel contains' is the wordplay.
'contains' indicates the answer is hidden within the clue.
'SALVE' can be found hidden inside 'colossal vessel'.
(Other definitions for salve that I've seen before include "Greeting in Rome" , "Relieve - ointment" , "Something that soothes" , "Ointment from the vales" , "Save - ointment" .)