Vile radical kid piercing eye (8)
I believe the answer is:
horrible
'vile' is the definition.
(thesaurus)
'radical kid piercing eye' is the wordplay.
'radical' becomes 'r' (chemistry abbreviation).
'kid' becomes 'rib' (both can mean to tease or mock).
'piercing' means one lot of letters goes inside another (some letters pierce a hole in a word).
'eye' becomes 'hole' (I've seen this in another clue).
'r'+'rib'='rrib'
'rrib' put within 'hole' is 'HORRIBLE'.
(Other definitions for horrible that I've seen before include "Abhorrent" , "Unkind" , "Dreadful, awful" , "'Ghastly, grisly (8)'" , "Nasty" .)