Walk across, having finished backing favourites (8)
I believe the answer is:
overstep
'walk across' is the definition.
(to step over something)
'finished backing favourites' is the wordplay.
'finished' becomes 'over' (similar in meaning).
'backing' says the letters should be written backwards.
'favourites' becomes 'pets' ('pet' can be a synonym of 'favourite').
'pets' in reverse letter order is 'step'.
'over'+'step'='OVERSTEP'
'having' acts as a link.
(Other definitions for overstep that I've seen before include "Exceed" , "Go beyond (a limit)" , "Pass beyond (a limit)" , "go too far" , "Transgress" .)