On which a policeman will show the way? (5,4)
I believe the answer is:
point duty
'on which a policeman' is the definition.
I can't tell whether this defines the answer.
'will show the way?' is the wordplay.
'will' means one lot of letters go next to another (I've seen this in other clues).
'show' becomes 'point' (synonyms).
'the way?' becomes 'duty' (I can't justify this - if you can you should give a lot more credence to this answer).
'point' next to 'duty' is 'POINT-DUTY'.
Can you help me to learn more?
(Other definitions for point duty that I've seen before include "part of policeman's job" , "job for the police" , "work done by a traffic controller" , "Traffic control by policeman for toy pundit" .)