Sister in on understanding (3)
I believe the answer is:
nun
'sister' is the definition.
(sister is a term of address for a nun)
'in on understanding' is the wordplay.
'in' says the answer is hidden in the clue.
'NUN' can be found hidden inside 'on understanding'.
(Other definitions for nun that I've seen before include "One in holy order" , "Religious sister" , "Member of religious order" , "Woman in religious order" , "Sister of mercy" .)