Expression used in Outer Mongolia (4)
I believe the answer is:
term
'expression' is the definition.
(I've seen this in other clues)
'in outer mongolia' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TERM' is hidden within 'outer mongolia'.
'used' acts as a link.
(Other definitions for term that I've seen before include "what being 9 [PREGNANT] leads to?" , "Fixed period of time" , "Hilary for one" , "expected time of delivery" , "Fixed period or special word" .)