First man to find a barrier (4)
I believe the answer is:
adam
'first man' is the definition.
(I've seen this before)
'a barrier' is the wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier).
'a'+'dam'='ADAM'
'to find' is the link.
(Other definitions for adam that I've seen before include "first in the race?" , "First man in the Bible" , "First guy" , "Fellow" , "who transformed Palladian architecture?" .)