Be critical of backing tipster's favourite (3)
I believe the answer is:
nap
'tipster's favourite' is the definition.
(favourite or predicted winner of a race)
'be critical of backing' is the wordplay.
'be critical of' becomes 'pan' (to pan is to criticise heavily).
'backing' says the letters should be written backwards.
'pan' written backwards gives 'NAP'.
(Other definitions for nap that I've seen before include "Raised surface of a fabric and short sleep" , "Little sleep" , "Raised fibres on surface of cloth or leather" , "Siesta" , "Short sleep on soft surface texture?" .)