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 wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier).
'to find' is the link.
(Other definitions for adam that I've seen before include "Biblical first man", "Fellow", "Father of man", "who transformed the Palladian style?", "the first orphan?", "The first person", "Father of Cain and Abel", "Male", "Early man" and "who transformed Palladian architecture?".)