Backbone of head of state seen before long (5)
I believe the answer is:
spine
'backbone' is the definition.
('spine' can be a synonym of 'backbone')
'head of state seen before long' is the wordplay.
'head of' says to take the initial letters.
'seen before' is a charade indicator (letters next to each other).
'long' becomes 'pine' ('pine' can be a synonym of 'long').
The initial letter of 'state' is 's'.
's'+'pine'='SPINE'
'of' is the link.
(Other definitions for spine that I've seen before include "Backbone" , "Strength of character" , "Series of vertebrae" , "Part of book's jacket" , "Line of vertebrae" .)