Insipidly sweet short message broken off (4)
I believe the answer is:
twee
'insipidly sweet' is the definition.
'short message broken off' is the wordplay.
'short message' becomes 'tweet'.
'broken off' means to remove the last letter (the word is interrupted before it finishes).
'tweet' with its last letter taken away is 'TWEE'.
(Other definitions for twee that I've seen before include "Affected quaint" , "Sentimentally pretty" , "Sentimental, quaint" , "Excessively quaint" , "Overly cute" .)