Prison officer (6)
'prison officer' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for warder that I've seen before include "screw" , "Prison officer" , "Reward for prison guard" , "One is in charge of prisoners" , "Jailer" .)
(Other definitions for warden that I've seen before include "Guard, keeper" , "Prison or traffic official" , "Head of certain schools, colleges, etc" , "Custodian" , "Person in charge (of traffic etc)" .)