Piece of music in 'Omen' maybe brought up as a reminder (8)
I believe the answer is:
mnemonic
'a reminder' is the definition.
(I've seen this before)
'piece of music in omen maybe brought up' is the wordplay.
'piece of' indicates a hidden word.
'brought up' says the letters should be written backwards (in a down clue, letters go up).
'cinomenm' can be found hidden inside 'music in omen maybe'.
'cinomenm' reversed gives 'MNEMONIC'.
'as' acts as a link.
(Other definitions for mnemonic that I've seen before include "jogger" , "Device for remembering things" , "It reminds us" , "Trigger for the memory" , "reminder" .)