Stow on unknown level (6)
I believe the answer is:
storey
'level' is the definition.
(synonyms)
'stow on unknown' is the wordplay.
'stow' becomes 'store' (synonyms).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'unknown' becomes 'y' (algebra).
'store'+'y'='STOREY'
(Other definitions for storey that I've seen before include "Floor or level in a building" , "section of building" , "Layer of house" , "Oyster (anag) -- level" , "part of the building" .)