Plan to win one test (5)
I believe the answer is:
pilot
'test' is the definition.
(synonyms)
'plan to win one' is the wordplay.
'plan' becomes 'plot' (plot is a kind of plan).
'to win' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'plot' placed around 'i' is 'PILOT'.
(Other definitions for pilot that I've seen before include "Guide - flier" , "Trial broadcast" , "One-off programme" , "Steer, test (scheme)" , "Controller of aircraft" .)