One of two executives is taken hostage, exciting rebel leaders (6)
I believe the answer is:
either
'one of two' is the definition.
(I've seen this in another clue)
'executives is taken hostage exciting rebel leaders' is the wordplay.
'leaders' indicates taking the first letters.
The initial letters of 'executives is taken hostage exciting rebel' is 'EITHER'.
(Other definitions for either that I've seen before include "Each of two; moreover" , "Word introducing first of two 16s" , "Word introducing first alternative" , "Choice of two" , "One of two alternatives" .)