One of the first in the phone book (5)
I believe the answer is:
aaron
'the phone book' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?
'one of the first in' is the wordplay.
'one' becomes 'a'.
'of' becomes 'ron' (I am not sure about this - if you are sure you should believe this answer much more).
'the first' becomes 'a'.
'in' means one lot of letters goes inside another.
'a'+'ron'='aron'
'aron' enclosing 'a' is 'AARON'.
Can you help me to learn more?
(Other definitions for aaron that I've seen before include "Moses's brother" , "- - Copland (mus.)" , "Brother of Moses, led the Jews to the Promised Land" , "Brother of Moses who had blessed rod" , "Bearded fellow" .)