Half of guys slip up badly, meeting a terrorist (9)
I believe the answer is:
guerrilla
'terrorist' is the definition.
(thesaurus)
'half of guys slip up badly meeting a' is the wordplay.
'half of' means to take half the letters.
'slip up' becomes 'err' (to err is to make an error).
'badly' becomes 'ill' (eg in ill-advised).
'meeting' is a charade indicator (letters next to each other).
'guys' cut in half is 'gu'.
'gu'+'err'+'ill'+'a'='GUERRILLA'
(Other definitions for guerrilla that I've seen before include "Resistance fighter" , "One fighting" , "Such tactics" , "Member of politically-motivated fighting group" , "Irregular bandit" .)