Aid to memory (8)
I believe the answer is:
mnemonic
'aid to memory' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for mnemonic that I've seen before include "That which helps memory, a rhyme say" , "Trigger for the memory" , "Letters aiding memory" , "Something by which to remember" , "It aids recall" .)