it's measured in degrees (11)
'it's measured in degrees' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for arc that I've seen before include "Part of a curve" , "Segment of circle" , "Curve of car" , "circle? Not entirely" , "Part of circle; discharge" .)
(Other definitions for temperature that I've seen before include "Measure of warmth" , "Measure of heat" , "Degree of heat" , "Degree of hotness" .)