Sister's name sure to be changed (5)
I believe the answer is:
nurse
'sister's' is the definition.
(I've seen this before)
'name sure to be changed' is the wordplay.
'name' becomes 'n' (common abbreviation e.g. on forms).
'to be changed' indicates an anagram.
'sure' is an anagram of 'urse'.
'n'+'urse'='NURSE'
(Other definitions for nurse that I've seen before include "Entangle" , "dogfish or > worker bee" , "Minister to" , "Feed" , "one working in theatre" .)