Youth is insignificant (5)
I believe the answer is:
minor
I believe this clue is a double definition.
'youth' is the first definition.
(thesaurus)
'insignificant' is the second definition.
('minor' can be similar in meaning to 'insignificant')
'is' acts as a link.
(Other definitions for minor that I've seen before include "One not yet of legal age" , "Paltry" , "Under-age or of little importance" , "Not major" , "Inconsequential" .)