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' says the letters should be written in reverse.
'in' indicates putting letters inside.
're' in reverse letter order is 'er'.
'gmans' going around 'er' is 'GERMANS'.

'the' acts as a link.