Legendary German singer teaching the French one (7)
I believe the answer is:
lorelei
'legendary german singer' is the definition.
(I've seen this in another clue)
'teaching the french one' is the wordplay.
'teaching' becomes 'lore' (lore is traditional knowledge or learning).
'the french' becomes 'le' ('the' in French).
'one' becomes 'i' (Roman numeral).
'lore'+'le'+'i'='LORELEI'
(Other definitions for lorelei that I've seen before include "A dangerously attractive lady" , "Fascinating women" , "Legendary siren and rock in the R Rhine" , "Legendary siren luring Rhine boatmen to destruction" , "Twenty down siren nymph; lie role (anag.)" .)