Preface perhaps key (6)
I believe the answer is:
opener
This might be a double definition.
'preface' is the first definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'key' is the second definition.
(I've seen this in another clue)
'perhaps' is the link.
Can you help me to learn more?
(Other definitions for opener that I've seen before include "First going in to bat" , "It may provide access to food" , "Batsman number one" , "Tins need one" , "Introductory remark" .)