First man to find a barrier (4)
I believe the answer is:
Here is my best explanation:
'first man' is the definition.
(I've seen this before)
'a barrier' is the subsidiary indication.
'barrier' becomes 'dam' (dam is a kind of barrier).
'to find' is the link.
(Other definitions for adam that I've seen before include "Eve's partner", "Eves husband", "- - Bede by George Eliot", "Father of Cain and Abel", "Male", "who transformed the Palladian style?", "Biblical first human", "Man on the ceiling of the Sistine Chapel", "Early man" and "__ Smith, author of The Wealth of Nations".)