In error altering instrument (8)
I believe the answer is:
triangle
'instrument' is the definition.
(triangle is a kind of instrument)
'in error altering' is the wordplay.
'in error' indicates an anagram.
'altering' anagrammed gives 'TRIANGLE'.
(Other definitions for triangle that I've seen before include "Simple percussion instrument" , "Figure - percussion instrument" , "Three-sided shape" , "involved in eternal emotional conflict" , "Three-sided percussion instrument" .)