Reforming EU, strive to keep Germany in thrall (9)
I believe the answer is:
servitude
'thrall' is the definition.
(I know that servitude is a type of thrall)
'reforming eu strive to keep germany' is the wordplay.
'reforming' indicates anagramming the letters.
'to keep' is an insertion indicator.
'germany' becomes 'd' (vehicle registration code for Deutschland).
'eu'+'strive'='eustrive'
'eustrive' with letters rearranged gives 'servitue'.
'servitue' going around 'd' is 'SERVITUDE'.
'in' is the link.
(Other definitions for servitude that I've seen before include "State of slavery" , "Bondage" .)