American who serves German artist (6)
I believe the answer is:
giotto
'artist' is the definition.
(14th-century Italian painter)
'american who serves german' is the wordplay.
'american who serves' becomes 'GI' (GI can informally mean a US soldier).
'german' becomes 'otto' (I've seen this before).
'gi'+'otto'='GIOTTO'
(Other definitions for giotto that I've seen before include "C thirteen-fourteen Italian painter" , "Fourteenth century Florentine artist" , "C14 Florentine frescoist" , "Italian painter, 1267-1337" , "Florentine painter" .)