Does it look about half an inch on the map? (5)
I believe the answer is:
seine
'half an inch on the map?' is the definition.
I can't tell whether this definition defines the answer.
'does it look about' is the wordplay.
'does it' becomes 'in' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'look' becomes 'see'.
'about' is an insertion indicator.
'in' placed into 'see' is 'SEINE'.
Can you help me to learn more?
(Other definitions for seine that I've seen before include "Parisian river" , "Second longest river in France" , "Paris's river" , "French river -- type of fishing net" , "Paris river" .)