One unique barrier broken down at first (4,4)
I believe the answer is:
rare bird
'one unique' is the definition.
(an unusual or exceptional individual)
'barrier broken down at first' is the wordplay.
'broken' indicates an anagram.
'at first' indicates taking the first letters.
The first letter of 'down' is 'd'.
'barrier' with letters rearranged gives 'rarebir'.
'rarebir'+'d'='RARE BIRD'
(Other definitions for rare bird that I've seen before include "Unusual type" , "Exceptional thing" , "Exceptional person" , "the talking point of twitchers?" .)