Source of poisoning? Induce discomfort with half of it (4,5)
I believe the answer is:
lead paint
'source of poisoning?' is the definition.
The definition suggests a singular noun which matches the answer.
'induce discomfort with half of it' is the wordplay.
'induce' becomes 'lead' (leading is a kind of inducing).
'discomfort with' becomes 'pain' (I can't justify this - if you can you should give a lot more credence to this answer).
'half of' suggests halving the letters.
'it' halved is 't'.
'lead'+'pain'+'t'='LEAD PAINT'
Can you help me to learn more?
(Other definitions for lead paint that I've seen before include "Old covering now considered hazardous" , "Decorative or protective coating with toxic metal base" , "colouring agent" .)