Leaders of strong parties, if necessary, exhibit backbone (5)
I believe the answer is:
spine
'backbone' is the definition.
('spine' can be a synonym of 'backbone')
'leaders of strong parties if necessary exhibit' is the wordplay.
'leaders of' suggests taking the first letters.
The initial letters of 'strong parties if necessary exhibit' is 'SPINE'.
(Other definitions for spine that I've seen before include "Thorn" , "Part of book's jacket" , "Line of vertebrae" , "Part of a book" , "Vertebral column" .)