Harmful material in Belgium found on street starting off and pocketed by fool (8)
I believe the answer is:
asbestos
'harmful material' is the definition.
'asbestos' can be an answer for 'material' (asbestos is a kind of material). I am not sure about the 'harmful' bit.
'belgium found on street starting off and pocketed by fool' is the wordplay.
I cannot really see how this works, but
'belgium' could be 'be' (I've seen this in other clues) and 'be' is found in the answer.
'street' could be 'st' (abbreviation) and 'st' is located in the answer.
This explanation may well be incorrect...
'in' acts as a link.
Can you help me to learn more?
(Other definitions for asbestos that I've seen before include "'Fireproof substance, dangerous to health (8)'" , "insulating material" , "Fireproof material, now known to be dangerous" , "Fibrous material, doesn't burn" , "Fire-resistant material, injurious to health" .)