Slight change of units across back of wall (6)
I believe the answer is:
insult
'slight' is the definition.
(to slight someone is to insult or disrespect them)
'change of units across back of wall' is the wordplay.
'change of' is an anagram indicator (the letter order is changed).
'across' is an insertion indicator.
'back of' says to take the final letters.
The final letter of 'wall' is 'l'.
'units' is an anagram of 'insut'.
'insut' going around 'l' is 'INSULT'.
(Other definitions for insult that I've seen before include "Slander" , "Treat with contempt" , "Abusive remark" , "Slight - affront" , "Treat with abuse" .)