Short message online is excessively sentimental (4)
I believe the answer is:
twee
'excessively sentimental' is the definition.
(twee can mean sentimental or quaint)
'short message online' is the wordplay.
'short' means to remove the last letter.
'message online' becomes 'tweet'.
'tweet' with its final letter taken away is 'TWEE'.
'is' acts as a link.
(Other definitions for twee that I've seen before include "Sickly sweet" , "Mawkish" , "Niminy-piminy" , "Over-pretty, sentimental" , "Sentimental, quaint" .)