Medic on track nursing rebel fighter (6)
I believe the answer is:
contra
'rebel fighter' is the definition.
(I've seen this before)
'medic on track nursing' is the wordplay.
'nursing' indicates a hidden word (the answer is held or nursed by the clue).
'CONTRA' is hidden in the letters of 'medic on track'.
(Other definitions for contra that I've seen before include "Against in argument" , "US-backed fighter" , "Rebel fighter" , "guerrilla" , "Con, Art could be against it" .)