Is tense after naughty child becomes radical (9)
I believe the answer is:
terrorist
'radical' is the definition.
(terrorist is a kind of radical)
'is tense after naughty child' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'after' is a charade indicator (letters next to each other).
'naughty child' becomes 'terror'.
'is'+'t'='ist'
'ist' after 'terror' is 'TERRORIST'.
'becomes' is the link.
(Other definitions for terrorist that I've seen before include "Bin Laden, maybe" , "User of violence for political ends" , "Fanatic" , "Political fighter" , "killer" .)