Promise to get collateral (6)
I believe the answer is:
pledge
This clue is a double definition.
'promise' is the first definition.
(pledging is a kind of promising)
'collateral' is the second definition.
(thesaurus)
'to get' is the link.
(Other definitions for pledge that I've seen before include "Plight" , "assurance" , "Deposit" , "Bond" , "Solemn undertaking" .)