The FBI agent's again turned in the Europeans (7)

'europeans' is the definition.
(German is an example)

'the fbi agent's again turned in' is the wordplay.
'the fbi agent's' becomes 'Gmans' ('G-man' can be a synonym of 'fbi agent').
'again' becomes 're' (prefix meaning 'again').
'turned' is a reversal indicator.
'in' means one lot of letters goes inside another.
're' back-to-front is 'er'.
'gmans' placed around 'er' is 'GERMANS'.

'the' acts as a link.