Period in Outer Mongolia (4)
I believe the answer is:
term
'period' is the definition.
(I know that term is a type of period)
'in outer mongolia' is the wordplay.
'in' says the answer is hidden in the clue.
'TERM' can be found hidden inside 'outer mongolia'.
(Other definitions for term that I've seen before include "Word or expression and period of time" , "Limit - school time" , "Time spent at school" , "Period of school life" , "Period of education" .)