Insecticide – source of dangerous mistakes involving iodine (6)

'insecticide' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'source of dangerous mistakes involving iodine' is the wordplay.
'source of' says to take the initial letters (the start - or source - of the word).
'mistakes' becomes 'errs' ('err' can be a synonym of 'mistake').
'involving' means one lot of letters goes inside another.
'iodine' becomes 'I' (I is the chemical symbol for iodine).
The initial letter of 'dangerous' is 'd'.
'errs' enclosing 'i' is 'erris'.
'd'+'erris'='DERRIS'

Can you help me to learn more?