Former power on the decline (9)
I believe the answer is:
preceding
'former' is the definition.
(thesaurus)
'power on the decline' is the wordplay.
'power' becomes 'p' (symbol in physics).
'on' becomes 're' ('on' can be mean 'with reference to').
'the decline' becomes 'ceding' (I can't justify this - if you can you should believe this answer much more).
'p'+'re'+'ceding'='PRECEDING'
Can you help me to learn more?
(Other definitions for preceding that I've seen before include "Coming/going before" , "Coming before" , "earlier" .)