There's nothing in backing a novice (3)
I believe the answer is:
nil
'there's nothing' is the definition.
(I've seen this before)
'in backing a novice' is the wordplay.
'backing' says the letters should be written backwards.
'a novice' becomes 'L' (abbreviation for Learner e.g. on UK cars).
'in' in reverse letter order is 'ni'.
'ni'+'l'='NIL'
(Other definitions for nil that I've seen before include "Score of nought" , "Nothing, naught" , "Nothing - in Latin" , "Lacking value" , "none" .)