One thing you don't want in the fire (3)
I believe the answer is:
fat
'fire' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'one thing you don't want in the' is the wordplay.
I cannot really see how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is found within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
A single letter 'f' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for fat that I've seen before include "Naturally oily substance" , "eg Lard" , "Excess body weight" , "Obese, having spare tyre etc" , "As Bunter" .)