A device to aid memory (8)
I believe the answer is:
mnemonic
'a device to aid memory' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mnemonic that I've seen before include "It reminds us" , "It aids recall" , "Trigger for the memory" , "Device to aid memory recall" , "Something by which to remember" .)