Insult is, for Macron, least significant (9)
I believe the answer is:
slightest
'significant' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?
'insult is for macron least' is the wordplay.
'insult' becomes ''slight' (I've seen this before).
'is for' means one lot of letters go next to another.
'macron least' becomes 'est' (I am not sure about this - if you are sure you should believe this answer much more).
'slight'+'est'='SLIGHTEST'
Can you help me to learn more?
(Other definitions for slightest that I've seen before include "Most tenuous or smallest" , "least important" , "carrying least weight?" , "Most tenuous or meagre" .)