I admit having no piercing may be stupid (8)
I believe the answer is:
ignorant
'be stupid' is the definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I can't see?
'i admit having no piercing' is the wordplay.
'admit' becomes 'grant'.
'having no' becomes 'no' (I've seen this in other clues).
'piercing' is an insertion indicator (some letters pierce a hole in others).
'grant' placed around 'no' is 'gnorant'.
'i'+'gnorant'='IGNORANT'
'may' is the link.
Can you help me to learn more?
(Other definitions for ignorant that I've seen before include "No rating (anag) - clueless" , "Rude or knowing very little" , "Lacking in education" , "Unknowing" , "Poorly-educated" .)