Factory ultimately supporting shop floor (6)
I believe the answer is:
storey
'floor' is the definition.
(US term for a floor in a building)
'factory ultimately supporting shop' is the wordplay.
'ultimately' says to take the final letters.
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'shop' becomes 'store' (synonyms).
The last letter of 'factory' is 'y'.
'y' put after 'store' is 'STOREY'.
(Other definitions for storey that I've seen before include "Layer in tall building" , "part of the building" , "Oyster (anag) -- level" , "floor of the House" , "section of building" .)