First man to make a barrier (4)
I believe the answer is:
adam
'first man' is the definition.
(first man in the Bible)
'a barrier' is the wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier).
'a'+'dam'='ADAM'
'to make' acts as a link.
(Other definitions for adam that I've seen before include "Biblical first human" , "Eve's partner" , "According to the Bible, progenitor of the human race" , "One of our common ancestors?" , "father figure" .)