Yearn for unmarried woman (4)
I believe the answer is:
miss
This clue is a double definition.
'yearn for' is the first definition.
(I've seen this before)
'unmarried woman' is the second definition.
(as opposed to Mrs)
(Other definitions for miss that I've seen before include "Don't hit" , "Fail to meet - girl" , "Teacher" , "failure" , "teacher's address" .)