Vow to turn up dressed in secretary's platforms (5)
I believe the answer is:
podia
'platforms' is the definition.
(podium is a kind of platform)
'vow to turn up dressed in secretary's' is the wordplay.
'vow' becomes 'I do' (I do - vow said at weddings).
'to turn up' is a reversal indicator.
'dressed in' is an insertion indicator (inserted letters are dressed in others).
'secretary' becomes 'PA' (abbreviation for personal assistant).
'ido' written backwards gives 'odi'.
'odi' placed inside 'pa' is 'PODIA'.
(Other definitions for podia that I've seen before include "Support structures for speakers" , "bases" , "Stands" , "Raised platforms" , "pedestals" .)