Win over Germany is a romp, oddly (6)
I believe the answer is:
disarm
'win over' is the definition.
(thesaurus)
'germany is a romp oddly' is the wordplay.
'germany' becomes 'd' (international vehicle registration code).
'oddly' means one should take alternating letters.
The alternating letters of 'romp' are 'rm'.
'd'+'is'+'a'+'rm'='DISARM'
(Other definitions for disarm that I've seen before include "Take away weapons or win over" , "Deprive of weapons or win over" , "Mollify or remove weapon from" , "Placate" , "Take away weapons or make less angry" .)