Fly: artifice required to squash one (5)
I believe the answer is:
pilot
'fly' is the definition.
(I know that fly can be written as pilot)
'artifice required to squash one' is the wordplay.
'artifice' becomes 'plot' (artifice can mean a trick or plot).
'required to squash' means one lot of letters goes inside another (some letters squash or squeeze in others).
'one' becomes 'i' (Roman numeral).
'plot' enclosing 'i' is 'PILOT'.
(Other definitions for pilot that I've seen before include "trial episode" , "Small flame that ignites a gas boiler" , "Navigator" , "One-off programme" , "who shows the way?" .)