Cancer or Capricorn (6)
'cancer or capricorn' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tropic that I've seen before include "Either that of Capricorn or Cancer", "Line of latitude determined by the solstice", "One of two parallels north and south of the equator", "Parallel of latitude about 25o N or S", "Capricorn, perhaps".)