Classic rum ball is reduced a tiny bit (5)
I believe the answer is:
crumb
'a tiny bit' is the definition.
(I've seen this in another clue)
'classic rum ball is reduced' is the wordplay.
'is reduced' says the answer is hidden in the clue (reducing the letters in the clue shows the answer).
'CRUMB' can be found hidden inside 'classic rum ball'.
(Other definitions for crumb that I've seen before include "Fragment of cake, bread" , "Very little comfort perhaps" , "Just a little bit" , "Tiny piece of bread or cake" , "Small bit (of comfort?)" .)