South American writes about the German underwear (10)

'underwear' is the definition.
(straps which hold up stockings)

'south american writes about the german' is the wordplay.
'south american' becomes 'sus' (I've seen this in other clues).
'writes' becomes 'pens' ('pen' can be a synonym of 'write').
'about' is an insertion indicator.
'the german' becomes 'der' ('the' in German).
'pens' placed around 'der' is 'penders'.
'sus'+'penders'='SUSPENDERS'