Subject takes right line (6)
I believe the answer is:
tropic
'line' is the definition.
(tropic is a kind of line)
'subject takes right' is the wordplay.
'subject' becomes 'topic' (as in a topic for discussion).
'takes' means one lot of letters goes inside another.
'right' becomes 'r' (common abbreviation).
'topic' placed around 'r' is 'TROPIC'.
(Other definitions for tropic that I've seen before include "One of two lines" , "eg Capricorn" , "Interzonal line" , "Line defining hot zone" , "Parallel of latitude about 25o N or S" .)