Spot the boil (6)
I believe the answer is:
seethe
'boil' is the definition.
('seethe' can be a synonym of 'boil')
'spot the' is the wordplay.
'spot' becomes 'see' (I've seen this before).
'see'+'the'='SEETHE'
(Other definitions for seethe that I've seen before include "Surge or foam, as if boiling" , "froth" , "(Of liquid) move as if boiling" , "Be silently angry" , "Boil; be intensely angry" .)