First man sees 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'
'sees' is the link.
(Other definitions for adam that I've seen before include "First guy" , "Faith, for example" , "Fellow" , "Eden inhabitant" , "Eves husband" .)