Distance seen in Tamil expanse (4)
I believe the answer is:
mile
'distance' is the definition.
(mile is a unit of distance)
'in tamil expanse' is the wordplay.
'in' says the answer is hidden in the clue.
'MILE' is hidden within 'tamil expanse'.
'seen' acts as a link.
(Other definitions for mile that I've seen before include "Imperial length" , "Stretch of road" , "5280 feet" , "Eight furlongs" , "Measure" .)