Assassin's proposal? (5)
I believe the answer is:
offer
I believe this is a double definition.
'assassin's' is the first definition.
(someone who 'offs' people)
'proposal?' is the second definition.
(an offer is a proposal or suggestion)
(Other definitions for offer that I've seen before include "Tender or propose" , "Present to or make a bid" , "Volunteer, present" , "killer?" , "Tender, make bid for" .)