Mostly underhand offer? (6)
I believe the answer is:
submit
'offer?' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for submit that I've seen before include "Defer to the will of another" , "Throw in the towel" , "Yield; argue" , "Yield to the control of another" , "Yield or refer to another" .)