Notice caps raised (4)
I believe the answer is:
spot
'notice' is the definition.
(spot can mean to notice or observe)
'caps raised' is the wordplay.
'caps' becomes 'tops' ('top' can be a synonym of 'cap').
'raised' is a reversal indicator (going up in a down clue).
'tops' backwards is 'SPOT'.
(Other definitions for spot that I've seen before include "jam > stain" , "Observe - locality" , "Fix" , "Informally, catch sight of" , "Notice - fictional puppy" .)