Some time in Outer Mongolia (4)
I believe the answer is:
term
'some time' is the definition.
(term can mean a period of time)
'in outer mongolia' is the wordplay.
'in' indicates a hidden word.
'TERM' can be found hidden inside 'outer mongolia'.
(Other definitions for term that I've seen before include "what being 9 [PREGNANT] leads to?" , "Word; limited period" , "Expression, or period in which law court holds sessions" , "Period of education" , "Limited period of time" .)