Nuts are replicable? Unique! (13)
I believe the answer is:
irreplaceable
'unique' is the definition.
(I've seen this before)
'nuts are replicable?' is the wordplay.
'nuts' is an anagram indicator (nuts can mean crazy).
'are'+'replicable'='arereplicable'
'arereplicable' anagrammed gives 'IRREPLACEABLE'.
(Other definitions for irreplaceable that I've seen before include "Without possible substitute" , "Cannot be substituted" , "precious!" , "Unique" , "Impossible to substitute for" .)