Short message not on outside of the box (7)
I believe the answer is:
notelet
'short message' is the definition.
(I've seen this before)
'not on outside of the box' is the wordplay.
'on outside of' is an insertion indicator.
'the box' becomes 'tele' (both can informally mean television).
'not' placed around 'tele' is 'NOTELET'.
(Other definitions for notelet that I've seen before include "Paper for informal letter" , "correspondence" , "Small sheet for letter" , "Decorative letter" , "Short folded letter" .)