Kinds of nasty pesticides (5)
I believe the answer is:
types
'kinds' is the definition.
(type is a kind of kind)
'of nasty pesticides' is the wordplay.
'of' indicates the answer is hidden within the clue.
'TYPES' can be found hidden inside 'nasty pesticides'.
(Other definitions for types that I've seen before include "Sorts; kinds" , "Uses a keyboard to write" , "Categories having common characteristics" , "Writes using a keyboard" , "Uses a keyboard; sorts" .)