Expression in Outer Mongolia (4)

I believe the answer is:

term

'expression' is the definition.
(thesaurus)

'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 "what being 9 [PREGNANT] leads to?" , "Period of education" , "Word; limited period" , "Time spent at school" , "Word or expression and period of time" .)

I've seen this clue in The Mirror.

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