Answer:

cosign

Answer	Crossword Clue
COSIGN	What a teen's parents might do for their lease
COSIGN	Share the dotted line
COSIGN	Assist in securing a loan

COSIGN	Assist in getting a loan
COSIGN	Back up, as a loan
COSIGN	Help to get a loan
COSIGN	Endorse with another.
COSIGN	Back up a loan
COSIGN	Assure payment of a promissory note
COSIGN	Back another's promissory note
COSIGN	Help to secure a loan, maybe

COSIGN	Dad's role in teen car purchase
COSIGN	Endorse for a loan, perhaps