Boil, or the spot on top (6)
I believe the answer is:
seethe
'boil' is the definition.
('seethe' can be a synonym of 'boil')
'the spot on top' is the wordplay.
'spot' becomes 'see' (I've seen this before).
'on top' means one lot of letters go next to another (down clues only).
'the' after 'see' is 'SEETHE'.
'or' acts as a link.
(Other definitions for seethe that I've seen before include "be incensed" , "Boil or become very angry" , "Feel intense rage" , "(Of liquid) boil" , "Boil; fume with suppressed rage" .)