Card on which a short message is permitted (7)
I believe the answer is:
notelet
'card on' is the definition.
'notelet' can be an answer for 'card' (a small card or paper for a note). I'm not sure about the 'on' bit.
'a short message is permitted' is the wordplay.
'a short message' becomes 'note' (I've seen this before).
'is permitted' becomes 'let' (to let something happen is to permit it).
'note'+'let'='NOTELET'
'which' is the link.
Can you help me to learn more?
(Other definitions for notelet that I've seen before include "informal communication" , "Small written communication" , "Decorative missive" , "Decorative letter" , "Paper folded" .)