Vow to turn up wearing secretary's platforms (5)
I believe the answer is:
podia
'platforms' is the definition.
(podium is a kind of platform)
'vow to turn up wearing secretary's' is the wordplay.
'vow' becomes 'I do' (I do - vow said at weddings).
'to turn up' shows that the letters should be reversed in order.
'wearing' is an insertion indicator.
'secretary' becomes 'PA' (abbreviation for personal assistant).
'ido' back-to-front is 'odi'.
'odi' going inside 'pa' is 'PODIA'.
(Other definitions for podia that I've seen before include "pedestals" , "Raised platforms" , "Support structures for speakers" , "bases" , "Stands" .)