Grecian's holding up decorator (4)
I believe the answer is:
icer
'decorator' is the definition.
(some decorating a cake with icing)
'grecian's holding up' is the wordplay.
'holding' indicates a hidden word.
'up' is a reversal indicator (in down clue: letters go upwards).
'reci' can be found hidden inside 'grecians'.
'reci' written backwards gives 'ICER'.
(Other definitions for icer that I've seen before include "One adding decoration, perhaps" , "Cake decorator" .)