Label, epic, rare, potentially unique (13)
I believe the answer is:
irreplaceable
'unique' is the definition.
(I've seen this before)
'label epic rare potentially' is the wordplay.
'potentially' is an anagram indicator (letters have the potential to spell a new word).
'label'+'epic'+'rare'='labelepicrare'
'labelepicrare' with letters rearranged gives 'IRREPLACEABLE'.
(Other definitions for irreplaceable that I've seen before include "Cannot be substituted" , "Without possible substitute" , "precious!" , "Whose loss cannot be made good" , "Impossible to substitute for" .)