Woman in a brown uniform (3)
I believe the answer is:
'woman' is the definition.
(I know that nun is a type of Christian nun)
'in a brown uniform' is the wordplay.
'in' indicates a hidden word.
'NUN' can be found hidden inside 'brown uniform'.
(Other definitions for nun that I've seen before include "She's made vows" , "A palindromic lady in religious order" , "religious person" , "Religious sister" , "Black and white pigeon" .)