First man's US president (5)
I believe the answer is:
adams
'us president' is the definition.
(I've seen this before)
'first man's' is the wordplay.
'first' becomes 'a' (first letter of the alphabet).
'man' becomes 'dams' (I can't justify this - if you can you should believe this answer much more).
'a'+'dams'='ADAMS'
Can you help me to learn more?
(Other definitions for adams that I've seen before include "John Quincy --, sixth US president" , "American composer" , "Sort of apple" , "Former US president" , "Name of two early US presidents" .)