To tease us up is sweet! (5)
I believe the answer is:
sugar
'is sweet' is the definition.
(thesaurus)
'to tease us up' is the wordplay.
'to tease' becomes 'rag' ('rag' can be a synonym of 'tease').
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'rag'+'us'='ragus'
'ragus' reversed gives 'SUGAR'.
(Other definitions for sugar that I've seen before include "Make more palatable" , "Sucrose" , "White crystalline sweetener" , "Plantation product" , "Sweet white substance" .)