Louis XIV discovered Germany's cruel (6)
I believe the answer is:
unkind
'cruel' is the definition.
(synonyms)
'louis xiv discovered germany's' is the wordplay.
'louis xiv' becomes 'sunking' (Louis XIV was nicknamed the Sun King).
'discovered' means to look at the middle letters.
'germany' becomes 'd' (international vehicle registration code).
The middle of 'sunking' is 'unkin'.
'unkin'+'d'='UNKIND'
(Other definitions for unkind that I've seen before include "Mean, pitiless" , "Thoughtless -- inhumane" , "Uncaring, nasty" , "beastly to others?" , "Cruel, harsh" .)