Subject: drawing in right line on map (6)
I believe the answer is:
tropic
'line on map' is the definition.
'subject drawing in right' is the wordplay.
'subject' becomes 'topic' (as in a topic for discussion).
'drawing in' indicates putting letters inside (draw in can mean to contain or trap).
'right' becomes 'r' (common abbreviation).
'topic' placed around 'r' is 'TROPIC'.
(Other definitions for tropic that I've seen before include "Capricorn, perhaps" , "Line at nearly 23 degrees N or S" , "Line bordering hot region" , "Parallel of latitude about 25o N or S" , "Cancer/Capricorn line" .)