Notice heads up (4)
I believe the answer is:
spot
'notice' is the definition.
(spot can mean to notice or observe)
'heads up' is the wordplay.
'heads' becomes 'tops'.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'tops' reversed gives 'SPOT'.
(Other definitions for spot that I've seen before include "See; round mark" , "Notice - place" , "Drop of rain" , "Catch sight of mark in pots" , "Identify" .)