Simple-sounding tool (5)
I believe the answer is:
plane
'tool' is the definition.
(I know that Plane is a tool)
'simple-sounding' is the wordplay.
'simple' becomes 'plain' (associated in meaning).
'sounding' shows a homophone (sound like).
'plain' is a homophone of 'PLANE'.
(Other definitions for plane that I've seen before include "This flier" , "Tree - figure - aircraft" , "Smooth" , "Tool for smoothing wood" , "Smoother" .)