That woman's protecting one next in line (4)
I believe the answer is:
heir
'next in line' is the definition.
'that woman's protecting one' is the wordplay.
'that woman' becomes 'her' (I've seen this before**).
'protecting' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'her' placed around 'i' is 'HEIR'.
(Other definitions for heir that I've seen before include "Successor, next in line" , "Acknowledged beneficiary" , "Person who inherits" , "Treasured family possession" , "One hoping to succeed" .)