Device to aid memory (8)
I believe the answer is:
mnemonic
'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 "That which helps memory, a rhyme say" , "System to improve memory" , "It reminds us" , "Letters aiding memory" , "Device to aid memory recall" .)