German wife and son outside Germany must be impostors (6)
I believe the answer is:
frauds
'impostors' is the definition.
('fraud' can be a synonym of 'impostor')
'german wife and son outside germany' is the wordplay.
'german wife' becomes 'frau' (I've seen this before).
'and' says to put letters next to each other.
'son' becomes 's'.
'outside' indicates putting letters inside.
'germany' becomes 'd' (international vehicle registration code).
'frau'+'s'='fraus'
'fraus' placed around 'd' is 'FRAUDS'.
'must be' is the link.
(Other definitions for frauds that I've seen before include "Cheaters and swindlers" , "Criminal deceptions" , "con merchants" .)