After losing case, tyrant has to employ advocate (7)
I believe the answer is:
espouse
'advocate' is the definition.
(thesaurus)
'after losing case tyrant has to employ' is the wordplay.
I cannot really see how this works, but
'employ' could be 'use' ('use' can be a synonym of 'employ') and 'use' is present in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for espouse that I've seen before include "Give support to - see soup (anag)" , "See opus to support a cause" , "Give support to, say a cause" , "Take in marriage" , "Champion or support, as with a cause" .)