Barbed prong finally penetrating judge of advanced years (6)
I believe the answer is:
jagged
'barbed' is the definition.
'prong finally penetrating judge of advanced years' is the wordplay.
'finally' indicates one should take the final letters.
'penetrating' is an insertion indicator.
'judge' becomes 'j'.
'of advanced years' becomes 'aged'.
The last letter of 'prong' is 'g'.
'j'+'aged'='jaged'
'g' inserted within 'jaged' is 'JAGGED'.
(Other definitions for jagged that I've seen before include "Sharp-edged" , "Notched; rough-edged; uneven" , "Having sharp uneven surface" , "Having uneven edge, knife or rock" , "Uneven and pointed" .)