Try and sue crooked sponsor (6)
I believe the answer is:
surety
'sponsor' is the definition.
(surety is a kind of sponsor)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for surety that I've seen before include "Security - guarantee" , "Semitic language" , "Money given as guarantee of action" , "Safeguard; certainty" , "Safeguard given against financial loss" .)