The first man to construct a barrier (4)
I believe the answer is:
adam
'the 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 construct' acts as a link.
(Other definitions for adam that I've seen before include "one might confuse with total stranger" , "Male" , "father of us all?" , "__ Smith, author of The Wealth of Nations" , "Eden inhabitant" .)