I had backed "The Love State" (5)
I believe the answer is:
idaho
'state' is the definition.
(US state)
'i had backed the love' is the wordplay.
'backed' says the letters should be written backwards.
'the love' becomes 'o' (love means zero in tennis).
'had' in reverse letter order is 'dah'.
'i'+'dah'+'o'='IDAHO'
(Other definitions for idaho that I've seen before include "NW state of USA" , "US state in the Rocky Mountains" , "Union member?" , "One of 22 Across, capital Boise" , "American state, capital Boise (5)" .)