Leave, having eaten the last bit of Christmas pudding (7)
I believe the answer is:
dessert
'pudding' is the definition.
(thesaurus)
'leave having eaten the last bit of christmas' is the wordplay.
'leave' becomes 'desert' (deserting is a kind of leaving).
'having eaten' is an insertion indicator.
'the last bit of' indicates one should take the final letters.
The last letter of 'christmas' is 's'.
'desert' placed around 's' is 'DESSERT'.
(Other definitions for dessert that I've seen before include "Sweet, last course of a meal" , "tart?" , "Pudding, sweet" , "Charlotte maybe" , "Christmas pudding maybe" .)