Thank you Skrej. I know the meaning of the sentence, just confused of the grammar as a nonnative speaker.
"A dream came true. A dream will come true. A dream comes true. " are all understandable. but "A dream come true"?
and how to understand this structure grammatically, "X is +a full sentence."?
Grammatically, it's a noun phrase, not a sentence. I'll borrow SoothingDave's reply in #5:
... "her dream (that had) come true"
The whole thing is a noun phrase -- "come true"(without "that had") is a participial phrase(*1) modifying "her dream".
So to answer your question, "X is +a full sentence", it's actually "X is + a noun phrase".
This might also help: (it agrees with what Ostap said in #7)
be a dream come true meaning, definition, what is be a dream come true: if something is a dream come true, it ha...: Learn more.
www.ldoceonline.com
(*1) some would call this a participial clause