Dear sir or madam (6)
I believe the answer is:
reader
'madam' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'dear sir' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'dear' is 'read' which is within the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'or' is the link.
Can you help me to learn more?
(Other definitions for reader that I've seen before include "Peruser of texts" , "One perusing book" , "Person perusing a book or newspaper" , "Publisher's selector" , "Kind of textbook - or its peruser" .)