Scrap old pieces covered in banned pesticide (7)
I believe the answer is:
oddment
'scrap' is the definition.
(synonyms)
'old pieces covered in banned pesticide' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'pieces' becomes 'men' (man can mean a chess piece).
'covered in' is an insertion indicator.
'banned pesticide' becomes 'ddt' (I can't explain this - if you can you should give a lot more credence to this answer).
'men' going into 'ddt' is 'ddment'.
'o'+'ddment'='ODDMENT'
Can you help me to learn more?
(Other definitions for oddment that I've seen before include "a bit spare" , "Remnant" , "Leftover piece" , "Leftover item" , "Just a bit left" .)