To back her – that's different (5)
I believe the answer is:
other
'different' is the definition.
(similar in meaning)
'to back her' is the wordplay.
'back' says the letters should be written in reverse.
'to' backwards is 'ot'.
'ot'+'her'='OTHER'
'that's' is the link.
(Other definitions for other that I've seen before include "Remaining" , "Different; alternate" , "Alternative - unrelated" , "Distinct" , "Second" .)