Love turning on light with gravity (7)
I believe the answer is:
nothing
'love' is the definition.
(love means zero in tennis)
'turning on light with gravity' is the wordplay.
'turning' is a reversal indicator.
'light' becomes 'thin' (similar in meaning).
'with' means one lot of letters go next to another.
'gravity' becomes 'g'.
'on' written backwards gives 'no'.
'no'+'thin'+'g'='NOTHING'
(Other definitions for nothing that I've seen before include "A void" , "Zero, nil" , "signifying love" , "Nought; of no value" , "Zero, naught" .)