Period of time in Outer Mongolia (4)
I believe the answer is:
term
'period of time' is the definition.
(term is a kind of period of time)
'in outer mongolia' is the wordplay.
'in' says the answer is hidden in the clue.
'TERM' is hidden within 'outer mongolia'.
(Other definitions for term that I've seen before include "Through which something has got to last" , "Call" , "An expression or period in office" , "Part of the school year" , "Word or phrase" .)