Message from over-sentimental time (5)
I believe the answer is:
'message' is the definition.
(type of online message)
'over-sentimental time' is the wordplay.
'over-sentimental' becomes 'twee'.
'time' becomes 't' (abbreviation).
'from' is the link.
(Other definitions for tweet that I've seen before include "Message from Trump?" , "Brief message" , "Note of a small bird" , "Online communication" , "Short online message" .)