Three-quarters of those in 17 down working in a station (6)
I believe the answer is:
porter
'three-quarters of those in 17 down working' is the definition.
I can't judge whether this definition defines the answer.
'in a station' is the wordplay.
'in' indicates putting letters inside.
'a' becomes 'per'.
'station' becomes 'ort' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'per' enclosing 'ort' is 'PORTER'.
Can you help me to learn more?
(Other definitions for porter that I've seen before include "Hotel luggage carrier" , "Doorkeeper; dark beer" , "Hotel bag-carrier or dark beer" , "One carries luggage" , "Hospital worker; beer" .)