Introduction of soft metal with nickel backing (4-2)
I believe the answer is:
lead-in
'introduction' is the definition.
(I've seen this before)
'soft metal with nickel backing' is the wordplay.
'soft metal' becomes 'lead' (I've seen this before).
'with' says to put letters next to each other.
'nickel' becomes 'Ni' (Ni is the chemical symbol for nickel).
'backing' shows that the letters should be reversed in order.
'ni' reversed gives 'in'.
'lead'+'in'='LEAD-IN'
'of' is the link.
(Other definitions for lead-in that I've seen before include "Introductory passage" , "Introduction, preamble" , "Introduction to subject" .)