At No.1, he may have no peer (6)
I believe the answer is:
opener
'peer' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'at no 1 he may have no' is the wordplay.
'at' says to put letters next to each other.
'no' becomes 'o' (resembles 0 - 'no' can mean 'zero').
'p he may' becomes 'peer' (I am not sure about this - if you are sure you should believe this answer much more).
'have' indicates putting letters inside.
'no' becomes 'n' (common abbreviation).
'peer' placed around 'n' is 'pener'.
'o'+'pener' is 'OPENER'.
Can you help me to learn more?
(Other definitions for opener that I've seen before include "First event in a series" , "First goal in game" , "leading batsman" , "introduction" , "See 1" .)