A barrier for first man (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'
'for' acts as a link.
(Other definitions for adam that I've seen before include "Man on the ceiling of the Sistine Chapel" , "Scots. architect brothers" , "the first orphan?" , "A mad sort of ancestor" , "Fellow" .)