Catch in an untenable position (3)
I believe the answer is:
nab
'catch' is the definition.
(I've seen this before)
'in an untenable position' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NAB' is hidden in the letters of 'untenable position'.
(Other definitions for nab that I've seen before include "nail" , "Seize quickly" , "Arrest (colloq.)" , "Take" , "Catch (a felon)" .)