Set of signs originally observed in parts of Cadiz (6)
I believe the answer is:
zodiac
'set of signs' is the definition.
(I've seen this in another clue)
'originally observed in parts of cadiz' is the wordplay.
'originally' indicates taking the first letters.
'in' indicates putting letters inside.
'parts of' indicates anagramming the letters.
The first letter of 'observed' is 'o'.
'cadiz' with letters rearranged gives 'zdiac'.
'o' going inside 'zdiac' is 'ZODIAC'.
(Other definitions for zodiac that I've seen before include "Star sign chart" , "Diagram of astrological signs" , "Twelve constellations together" , "Heavenly belt of star-signs" , "nd of sky divided into twelve parts" .)