Caution — a PH that covers flammable oil (7)
I believe the answer is:
naphtha
'flammable oil' is the definition.
(I've seen this before)
'caution a ph that covers' is the wordplay.
'covers' indicates the answer is hidden within the clue (I've seen 'covered' mean this (the answer is covered up by the clue)).
'NAPHTHA' is hidden in the letters of 'caution a ph that'.
(Other definitions for naphtha that I've seen before include "Oil obtained from coal; hath pan (anag.)" , "Flammable liquid hydrocarbon mixture" , "Flammable oil from wood, coal tar, etc" , "Flammable liquid used as solvent" , "Rock-oil" .)