First man meets a barrier (4)
I believe the answer is:
adam
'first man' is the definition.
(first man in the Bible)
'meets a barrier' is the wordplay.
'meets' is a charade indicator (letters next to each other).
'barrier' becomes 'dam' (dam is a kind of barrier).
'a' next to 'dam' is 'ADAM'.
(Other definitions for adam that I've seen before include "According to the Bible, progenitor of the human race" , "Scottish architect, d.1792" , "First fruit-eater" , "A mad sort of ancestor" , "Man on the ceiling of the Sistine Chapel" .)