Good person, a German author (5)
I believe the answer is:
stein
'author' is the definition.
(Gertrude Stein)
'good person a german' is the wordplay.
'good person' becomes 'st' (saint).
'a german' becomes 'ein' ('a' in German).
'st'+'ein'='STEIN'
(Other definitions for stein that I've seen before include "Large earthenware beer mug" , "Inset (anag.)" , "Gertrude ---; US writer" , "Gertrude __, avant-garde US author, d. 1946" , "Beer-mug" .)