Killer of insects — one by mechanical process getting any number (8)
I believe the answer is:
rotenone
'killer of insects' is the definition.
(rotenone is a pesticide)
'one by mechanical process getting any number' is the wordplay.
'by' means one lot of letters go next to another.
'mechanical process' becomes 'rote' (as in learning by rote).
'getting' means one lot of letters go next to another.
'any number' becomes 'n' (I've seen this in other clues).
'rote'+'n'='roten'
'one' put after 'roten' is 'ROTENONE'.
(Another definition for rotenone that I've seen is " Insecticide".)