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' indicates taking half.
'slip up' becomes 'err' (to err is to make an error).
'badly' becomes 'ill' ('ill' can be adverbial as in 'ill-advised').
'meeting' says to put 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" .)