Test bed involves one (5)
I believe the answer is:
pilot
'test' is the definition.
('pilot' can be a synonym of 'test')
'bed involves one' is the wordplay.
'bed' becomes 'plot' (I've seen this before).
'involves' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'plot' going around 'i' is 'PILOT'.
(Other definitions for pilot that I've seen before include "Navigate" , "Steer (a ship or plane)" , "-- Whale; -- light" , "One licensed to fly" , "Small flame that ignites a gas boiler" .)