Woman in tribunal (3)
I believe the answer is:
una
'woman' is the definition.
'in tribunal' is the wordplay.
'in' indicates the answer is hidden within the clue.
'UNA' is hidden within 'tribunal'.
(Other definitions for una that I've seen before include "of satellite with no sides" , "A single Spanish" , "Diego's one" , "Girl's name; indefinite article (f. Italian)" , "- Stubbs, Eng. actress" .)