Baby Jesus . . . ultimately kid you rejected (5)
I believe the answer is:
sugar
'baby' is the definition.
(thesaurus)
'jesus ultimately kid you rejected' is the wordplay.
'ultimately' indicates one should take the final letters.
'kid' becomes 'rag' (both can mean to tease).
'you' becomes 'u' (commonly used in online communication).
'rejected' is a reversal indicator.
The last letter of 'jesus' is 's'.
'rag'+'u'='ragu'
'ragu' reversed gives 'ugar'.
's'+'ugar'='SUGAR'
(Other definitions for sugar that I've seen before include "Sweet ingredient" , "Argus (anag.)" , "Plantation product" , "Damn" , "White crystalline carbohydrate" .)