German agent turned by Yankee traveller (5)
I believe the answer is:
gypsy
'traveller' is the definition.
(gypsy is a kind of traveller)
'german agent turned by yankee' is the wordplay.
'german' becomes 'g' (abbreviation).
'agent' becomes 'spy' (spy is a kind of agent).
'turned' says the letters should be written backwards.
'by' means one lot of letters go next to another.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'spy' backwards is 'yps'.
'g'+'yps'+'y'='GYPSY'
(Other definitions for gypsy that I've seen before include "Member of a normadic people" , "old Broadway play" , "Traveller" , "Romany" .)