Favourites upset by the pace? (4)
I believe the answer is:
step
'the pace?' is the definition.
(pace is a kind of step)
'favourites upset' is the wordplay.
'favourites' becomes 'pets' ('pet' can be a synonym of 'favourite').
'upset' shows that the letters should be reversed in order.
'pets' reversed gives 'STEP'.
'by' is the link.
(Other definitions for step that I've seen before include "'Stage, rung (4)'" , "Ladder rung; pace" , "Measure taken" , "Footprint" , "Pace (taken one at a time?)" .)