Petty Officer to help put up platforms (5)
I believe the answer is:
podia
'platforms' is the definition.
(podium is a kind of platform)
'petty officer to help put up' is the wordplay.
'petty officer' becomes 'po' (naval abbreviation).
'to help' becomes 'aid' (synonyms).
'put up' shows that the letters should be reversed in order.
'aid' in reverse letter order is 'dia'.
'po'+'dia'='PODIA'
(Other definitions for podia that I've seen before include "bases" , "Stands" , "Support structures for speakers" , "Raised platforms" , "pedestals" .)