One had been winning but took it easy (5)
I believe the answer is:
idled
'took it easy' is the definition.
(I've seen this in another clue)
'one had been' is the wordplay.
'one had' becomes 'Id' ('I'd' can mean 'one had').
'been' becomes 'led' (leading is a kind of being).
'id'+'led'='IDLED'
'winning but' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for idled that I've seen before include "sat back" , "not worked" , "Spent time doing nothing" , "relaxed" , "Of engine, just ticked over" .)