Period of time in Chad, a year (3)
I believe the answer is:
day
'period of time' is the definition.
(day is a kind of period of time)
'in chad a year' is the wordplay.
'in' says the answer is hidden in the clue.
'DAY' is hidden in the letters of 'chad a year'.
(Other definitions for day that I've seen before include "Particular date" , "24-hour period" , "Period of time" , "Twenty-four hours" , "Lewis as poet" .)