Power of left constantly on decline (8)
I believe the answer is:
leverage
'power' is the definition.
(thesaurus)
'left constantly on decline' is the wordplay.
'left' becomes 'l' (common abbreviation).
'constantly' becomes 'ever' (I've seen this before).
'on' says to put letters next to each other.
'decline' becomes 'age' (I've seen this in other clues).
'l'+'ever'+'age'='LEVERAGE'
'of' is the link.
(Other definitions for leverage that I've seen before include "Power -- advantage" , "Mechanical advantage; influence" , "Purchase" , "Power to act effectively" , "can achieve power" .)