Swill with some particular insecticide (5)
I believe the answer is:
rinse
'swill' is the definition.
(I've seen this before)
'with some particular insecticide' is the wordplay.
'with' indicates a hidden word.
'RINSE' can be found hidden inside 'some particular insecticide'.
(Other definitions for rinse that I've seen before include "Flush" , "Wash in clean water" , "Wash off surface dirt" , "Run water through" , "Wash - siren (anag)" .)