Given the following tables and no possibility of changing them

CREATE TABLE CAR (
ID NUMBER (10),
NAME VARCHAR (50) NOT NULL,
PRIMARY KEY (ID)
);

CREATE TABLE PART (
CAR_ID NUMBER (10),
SEQUENCE NUMBER(3),
NAME VARCHAR (50),
PRIMARY KEY (CAR_ID, SEQUENCE),
FOREIGN KEY (CAR_ID) REFERENCES CAR(ID)
);

What is the most effective mapping? I want value semantics on PART (i.e. when I delete a CAR, I want all associated PARTs deleted as well). It seems composite-element mappings only allow you to use the primary key of the parent as the child's primary key (or perhaps I'm just mapping it wrong - a real possibility).

It looks like you've got a one to many relationship set up just fine. The problem I've found as a newcomer to Hibernate is that if you get away from a easily defined "id" in your entity then mapping things seems to become exponentially harder.

In my entities always using <id> (usually a simple integer sequence or auto-increment in MySQL) as well as a <natural-id>. Anyway, since you can't change PARTS I'd consider making that a component instead of an entity.

Maybe a mapping in your Car entity like this might work... (I haven't tested it so my XML might be a little off)