Plan to accept one as experimental (5)
I believe the answer is:
pilot
'experimental' is the definition.
(synonyms)
'plan to accept one' is the wordplay.
'plan' becomes 'plot' (plotting is a kind of planning).
'to accept' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'plot' placed around 'i' is 'PILOT'.
'as' acts as a link.
(Other definitions for pilot that I've seen before include "Name of Mr Rochester's dog" , "Plane flier" , "Controller of aircraft" , "This person can be found in a plane or a boat" , "Steer (a ship or plane)" .)