Accelerate when favourite's back in the lead (4,2)
I believe the answer is:
step up
'accelerate' is the definition.
(I've seen this before)
'favourite's back in the lead' is the wordplay.
'favourite's' becomes 'pet's' ('pet' can be a synonym of 'favourite').
'back' is a reversal indicator.
'in the lead' becomes 'up' (similar in meaning).
'pets' written backwards gives 'step'.
'step'+'up'='STEP-UP'
'when' acts as a link.
(Other definitions for step up that I've seen before include "Come forward" , "- Bezukhov (War and Peace)" , "Accelerate - promotion" , "To be promoted" , "Approach" .)