Medal keeps one operational (5)
I believe the answer is:
going
'operational' is the definition.
(thesaurus)
'medal keeps one' is the wordplay.
'medal' becomes 'gong' (informal term for a medal or prize).
'keeps' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'gong' going around 'i' is 'GOING'.
(Other definitions for going that I've seen before include "Disappearing" , "Failing to stay" , "Departure; current (rate)" , "Moving away, departing" , "on the way out" .)