SA resident ambassador seized by gunmen (4)
I believe the answer is:
rhea
'sa resident' is the definition.
(species of bird native to South America)
'ambassador seized by gunmen' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'seized by' is an insertion indicator.
'gunmen' becomes 'RA' (Royal Artillery).
'he' going into 'ra' is 'RHEA'.
(Other definitions for rhea that I've seen before include "It can't fly" , "S. Amer. flightless bird" , "Fertility goddess in ancient Greek mythology" , "South American flightless bird - mother of Zeus" , "Fast runner" .)