China ominously detains girl (5)
I believe the answer is:
naomi
'girl' is the definition.
(girl's name)
'china ominously detains' is the wordplay.
'detains' says the answer is hidden in the clue (the answer is detained by the clue).
'NAOMI' can be found hidden inside 'china ominously'.
(Other definitions for naomi that I've seen before include "Do I moan about Biblical Ruth's mother-in-law?" , "OT female" , "Mother-in-law of Ruth (Bible)" , "Girl's name, meaning "pleasant"" , "Girl's name -- I moan (anag)" .)