Clue: Flammable liquid ruined hat after hot vessel tipped over (7)
Answer: NAPHTHA
Explanation: Definition: Flammable liquid
A charade of NAPH, a reversal ('tipped over') of H ('hot') plus PAN ('vessel'); plus THA, an anagram ('ruined') of 'hat'. I came to this with the crossers …H.H. which looked wildly unlikely, but there it is.