Stern Unitarian keeping woman in order (3)
I believe the answer is:
nun
'woman in order' is the definition.
(I've seen this before)
'stern unitarian keeping' is the wordplay.
'keeping' says the answer is hidden in the clue.
'NUN' can be found hidden inside 'stern unitarian'.
(Other definitions for nun that I've seen before include "One in holy order" , "Devout woman" , "Sister of mercy" , "Carmelite, for example" , "Mother Teresa, maybe" .)