Excessively sentimental message? Not entirely (4)
I believe the answer is:
twee
'excessively sentimental' is the definition.
(twee can mean sentimental or quaint)
'message? not entirely' is the wordplay.
'message?' becomes 'tweet' (type of online message).
'not entirely' means to remove the last letter.
'tweet' with its final letter taken away is 'TWEE'.
(Other definitions for twee that I've seen before include "Niminy-piminy" , "Sentimentally pretty" , "Overly cute" , "Affectedly dainty or quaint" , "Small and pretty" .)