No longer having power, the French people — reason? (11)
I believe the answer is:
explanation
'reason?' is the definition.
(reason is a kind of explanation)
'no longer having power the french people' is the wordplay.
'no longer' becomes 'ex' (prefix meaning former or onetime).
'having' says to put letters next to each other.
'power' becomes 'p' (symbol in physics).
'the french' becomes 'la' ('the' in French).
'people' becomes 'nation' ().
'ex'+'p'+'la'+'nation'='EXPLANATION'
(Other definitions for explanation that I've seen before include "That which makes meaning clear" , "Justification for an action or belief" , "Reason or justification (for something)" , "Statement making something clear" .)