Subject has registered some latitude (6)
I believe the answer is:
tropic
'latitude' is the definition.
(I know that tropic is a type of latitude)
'subject has registered some' is the wordplay.
'subject' becomes 'topic' (as in a topic for discussion).
'has' indicates putting letters inside.
'some' indicates taking the first letters.
The initial letter of 'registered' is 'r'.
'topic' placed around 'r' is 'TROPIC'.
(Other definitions for tropic that I've seen before include "One of two lines" , "Either that of Capricorn or Cancer" , "Latitude 23 and a quarter 26 N or S" , "Parallel of latitude about 25o N or S" , "Line defining hot zone" .)