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 the answer is hidden within the clue.
'TERM' is hidden within '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" .)