Make a botch of altering instrument (8)
I believe the answer is:
triangle
'instrument' is the definition.
(triangle is a kind of instrument)
'make a botch of altering' is the wordplay.
'make a botch of' indicates anagramming the letters.
'altering' is an anagram of 'TRIANGLE'.
(Other definitions for triangle that I've seen before include "Integral (anag) - figure" , "Integral part of common geometric figure" , "diamond cut in half?" , "Three-sided figure - musical instrument" , "Percussion instrument of geometric origin" .)