Introduction, preamble (4-2)
I believe the answer is:
lead-in
I believe this is a double definition.
'introduction' is the first definition.
(I've seen this before)
'preamble' is the second definition.
(I've seen this before)
(Other definitions for lead-in that I've seen before include "Introductory passage" , "Introduction, preamble" , "Introduction to subject" .)