An indicator of intense rivalry (6)
I believe the answer is:
needle
I believe this is a double definition.
'an indicator' is the first definition.
(needle is a kind of indicator)
'intense rivalry' is the second definition.
(informal term for hostility arising from rivalry)
'of' is the link.
(Other definitions for needle that I've seen before include "Irritate, goad" , "Persistently annoy" , "Gramophone record stylus" , "Goad, provoke" , "Sewing item" .)