Turning up in mail: a German’s ceremonial robes (7)
I believe the answer is:
regalia
'ceremonial robes' is the definition.
(I've seen this before)
'turning up in mail a german's' is the wordplay.
'turning up' is a reversal indicator.
'in' indicates a hidden word.
'ailager' can be found hidden inside 'mail a germans'.
'ailager' reversed gives 'REGALIA'.
(Other definitions for regalia that I've seen before include "Emblems of royalty used at coronations" , "Finery" , "ceremonial gear" , "Trappings of royalty" , "Crown, sceptre etc used at a coronation" .)