Power of Atlas's heart always on decline (8)
I believe the answer is:
leverage
'power' is the definition.
(thesaurus)
'atlas's heart always on decline' is the wordplay.
'heart' says to take the centre.
'always' becomes 'ever' ('ever' can be a synonym of 'always'**).
'on' says to put letters next to each other.
'decline' becomes 'age' (I've seen this in other clues).
The middle of 'atlas' is 'l'.
'l'+'ever'+'age'='LEVERAGE'
'of' is the link.
(Other definitions for leverage that I've seen before include "Mechanical advantage; influence" , "Economic clout" , "Strategic advantage; large Eve (anag.)" , "financial advantage" , "can achieve power" .)