A warning on top of the brow (8)
I believe the answer is:
forehead
'the brow' is the definition.
('forehead' can be a synonym of 'brow')
'a warning on top' is the wordplay.
'a warning' becomes 'fore' (I've seen this before).
'on' means one lot of letters go next to another.
'top' becomes 'head' (head is a kind of top**).
'fore'+'head'='FOREHEAD'
'of' acts as a link.
(Other definitions for forehead that I've seen before include "Frontal bone" , "Space above the eyes" , "Front of the head" , "Brow" , "Upper part of the face" .)