Spot on the boil (6)
I believe the answer is:
seethe
'boil' is the definition.
('seethe' can be a synonym of 'boil')
'spot on the' is the wordplay.
'spot' becomes 'see' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'see'+'the'='SEETHE'
(Other definitions for seethe that I've seen before include "(Of liquid) boil" , "(Of liquid) move as if boiling" , "Bubble gently just below boiling point" , "be incensed" , "Feel intense rage" .)