Send a message or aid, possibly (5)
I believe the answer is:
radio
'send a message' is the definition.
(I've seen this before)
'or aid possibly' is the wordplay.
'possibly' is an anagram indicator.
'or'+'aid'='oraid'
'oraid' anagrammed gives 'RADIO'.
(Other definitions for radio that I've seen before include "Transmit" , "Receiving device" , "Broadcast medium" , "Electronic medium of communication" , "Broadcaster" .)