Power of Labour leader always preceding decline (8)
I believe the answer is:
leverage
'power' is the definition.
(thesaurus)
'labour leader always preceding decline' is the wordplay.
'leader' suggests taking the first letters.
'always' becomes 'ever' ('ever' can be a synonym of 'always').
'preceding' is a charade indicator (letters next to each other).
'decline' becomes 'age' (I've seen this in other clues).
The initial letter of 'labour' is 'l'.
'l'+'ever'+'age'='LEVERAGE'
'of' acts as a link.
(Other definitions for leverage that I've seen before include "What crowbar provides" , "Power of achievement; (US) gearing" , "Power -- advantage" , "Capacity to exert pressure" , "Economic clout" .)