Defeat admitted in umpteenth war trial (6)

I believe the answer is:
thwart
'defeat admitted in umpteenth war trial' is the definition.
'thwart' can be an answer for 'defeat' (I've seen this in another clue). I am not certain of the 'admitted in umpteenth war trial' bit.
'in umpteenth war trial' is the wordplay.
'in' indicates a hidden word.
'THWART' can be found hidden inside 'umpteenth war trial'.
Can you help me to learn more?
(Other definitions for thwart that I've seen before include "Prevent from succeeding" , "cross" , "Oppose successfully" , "Defeat" , "Frustrate or foil" .)
