One hundred and one becoming ninety-nine? (4)
I believe the answer is:
cone
'ninety-nine?' is the definition.
(as in a 99 Flake ice cream)
'one hundred and one becoming' is the wordplay.
'one hundred' becomes 'c' (Roman numeral).
'and' says to put letters next to each other.
'becoming' indicates an anagram.
'c' after 'one' is 'onec'.
'onec' is an anagram of 'CONE'.
(Other definitions for cone that I've seen before include "Volcano peak" , "Tapering solid" , "Road works marker" , "Tapering shape" , "often full of ice" .)