Action taken to back favourites (4)
I believe the answer is:
step
'action taken' is the definition.
'back favourites' is the wordplay.
'back' says the letters should be written backwards.
'favourites' becomes 'pets' ('pet' can be a synonym of 'favourite').
'pets' back-to-front is 'STEP'.
'to' acts as a link.
(Other definitions for step that I've seen before include "Abrupt change; pace" , "Put foot on a pest" , "Set foot down on pest" , "Second half of quick 20?" , "Tramp" .)