Post message that's short and sentimental (4)
I believe the answer is:
twee
'sentimental' is the definition.
(I've seen this before)
'post message that's short' is the wordplay.
'post message' becomes 't' (I am not sure about this - if you are sure you should believe this answer much more).
'that's short' becomes 'wee' (I've seen this in other clues).
't'+'wee'='TWEE'
'and' is the link.
Can you help me to learn more?
(Other definitions for twee that I've seen before include "Sentimental, quaint" , "Excessively cute" , "rather quaint" , "Overly cute" , "Excessively sweet" .)