Trivial insult ignored by son (5)
I believe the answer is:
light
'trivial' is the definition.
(thesaurus)
'insult ignored by son' is the wordplay.
'insult' becomes 'slight' (I've seen this before).
'ignored by' is a deletion indicator.
'son' becomes 's'.
'slight' with 's' removed is 'LIGHT'.
(Other definitions for light that I've seen before include "Not too demanding" , "Not dark; not heavy" , "dawn" , "Pale" , "Airy; not heavy" .)