Snack that's prepared ... I bet! (4)
I believe the answer is:
bite
'snack' is the definition.
(I know that snack can be written as bite)
'prepared i bet' is the wordplay.
'prepared' indicates anagramming the letters.
'i'+'bet'='ibet'
'ibet' is an anagram of 'BITE'.
'that's' acts as a link.
(Other definitions for bite that I've seen before include "Small amount of food" , "Small amount to eat" , "The taking of bait by a fish" , "Morsel" , "Take effect; snack" .)