Doctor protects a witness (9)
I believe the answer is:
spectator
'witness' is the definition.
(I know that witness can be written as spectator)
'doctor protects a' is the wordplay.
'doctor' indicates anagramming the letters.
'protects'+'a'='protectsa'
'protectsa' is an anagram of 'SPECTATOR'.
(Other definitions for spectator that I've seen before include "One watching a game" , "one found at ground?" , "weekly publication" , "given weekly inspection?" , "fan" .)