New ruse devised by sister (5)
I believe the answer is:
nurse
'sister' is the definition.
(I've seen this before)
'new ruse devised by' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'devised' indicates anagramming the letters.
'by' is a charade indicator (letters next to each other).
'ruse' with letters rearranged gives 'urse'.
'n'+'urse'='NURSE'
(Other definitions for nurse that I've seen before include "Tend - care for" , "Care for - harbour" , "Trained carer" , "Minister to" , "Patient tender" .)