It may be dropped in a short communication (4)
I believe the answer is:
line
This clue could be a double definition.
'it may be dropped' is the first definition.
(I've seen this in another clue)
'short communication' is the second definition.
'line' can be an answer for 'communication' (line is a kind of communication). I'm unsure of the 'short' bit.
'in a' acts as a link.
Can you help me to learn more?
(Other definitions for line that I've seen before include "Long narrow mark" , "Formation of one beside another" , "short note" , "business" , "cover on the inside" .)