One rice pudding is colder (5)
I believe the answer is:
icier
'colder' is the definition.
('icy' can be similar in meaning to 'cold')
'one rice pudding' is the wordplay.
'one' becomes 'i' (Roman numeral).
'pudding' indicates anagramming the letters (a mixture or pudding of the letters).
'rice' anagrammed gives 'cier'.
'i'+'cier'='ICIER'
'is' acts as a link.
(Other definitions for icier that I've seen before include "More reserved" , "More glacial" , "Even more frozen" , "More cold - really freezing" , "increasingly frigid" .)