100 - 1 = 99? (4)
I believe the answer is:
cone
'99?' is the definition.
(99 Flake is an ice cream product with a cone)
'100 1' is the wordplay.
'o00' becomes 'C' (Roman numeral).
'o' becomes 'one' (synonyms).
'c'+'one'='CONE'
(Other definitions for cone that I've seen before include "Roadworks marker" , "Volcano peak" , "Tapering solid" , "99, maybe" , "Edible ice cream container" .)