Former head not the front-runner (4)
I believe the answer is:
once
'former' is the definition.
(synonyms)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for once that I've seen before include "At some time past" , "One-time musical film and song" , "In times past" , "Some time back" , "One-time - one time" .)