A vessel; but when jammed, it's a car! (3)
I believe the answer is:
jar
'a vessel' is the definition.
(type of container)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for jar that I've seen before include "Container; be incongruous" , "Glass of beer (colloq.)" , "Storage container" , "Shock; container" , "Pint glass of beer, perhaps" .)