Tenacious rep isn't set in error (10)
I believe the answer is:
persistent
'tenacious' is the definition.
(I know that tenacious can be written as persistent)
'rep isn't set in error' is the wordplay.
'in error' is an anagram indicator.
'rep'+'isnt'+'set'='repisntset'
'repisntset' with letters rearranged gives 'PERSISTENT'.
(Other definitions for persistent that I've seen before include "Relentless" , "hanging on" , "Resolute - interminable" , "Determined" , "Continual" .)