Mawkish message not finished (4)
I believe the answer is:
twee
'mawkish' is the definition.
(I've seen this before)
'message not finished' is the wordplay.
'message' becomes 'tweet' (type of online message).
'not finished' means to remove the last letter.
'tweet' with its final letter taken off is 'TWEE'.
(Other definitions for twee that I've seen before include "Affectedly cute" , "Excessively cute" , "Pretty-pretty" , "Niminy-piminy" , "Overly cute" .)