Second favourite backed for pace (4)
I believe the answer is:
step
'for pace' is the definition.
(synonyms)
'second favourite backed' is the wordplay.
'second' becomes 's' (common abbreviation).
'favourite' becomes 'pet' (synonyms).
'backed' says the letters should be written backwards.
'pet' back-to-front is 'tep'.
's'+'tep'='STEP'
(Other definitions for step that I've seen before include "Section of flight" , "Part of a stairway" , "Pest (anag.)" , "Pace - part of a procedure" , "Put foot on a pest" .)