Slogan of many a German (5)
I believe the answer is:
motto
'slogan' is the definition.
(synonyms)
'many a german' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'a german' becomes 'otto' (common German name).
'm'+'otto'='MOTTO'
'of' acts as a link.
(Other definitions for motto that I've seen before include "Phrase in a Christmas cracker" , "Phrase to express character" , "Guiding adage" , "It may be proverbial" , "(Family) maxim" .)