Thick flammable liquid (3)
I believe the answer is:
tar
'thick flammable liquid' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tar that I've seen before include "Jack < to blacken" , "Dark viscous flammable substance" , "Dark liquid used on roads" , "Sailor and dressing for roads" , "used by those who pay to work on deck" .)