Fundamental sect loses leader and one partner (8)
I believe the answer is:
ultimate
'fundamental' is the definition.
('ultimate' can be a synonym of 'fundamental')
'sect loses leader and one partner' is the wordplay.
'sect' becomes 'cult' (I've seen this in other clues).
'loses leader' means to remove the first letter.
'and' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'partner' becomes 'mate' ('mate' can be a synonym of 'partner'**).
'cult' with its initial letter taken away is 'ult'.
'ult'+'i'+'mate'='ULTIMATE'
(Other definitions for ultimate that I've seen before include "Last, final or extreme" , "Best or most extreme" , "Final; fundamental" , "Last or concluding element" , "Happening at the end of a process, final" .)