A series of bike races involving fields in any event (2,5)
I believe the answer is:
at least
'in any event' is the definition.
(I've seen this in another clue)
'a series of bike races involving fields' is the wordplay.
'a series of bike races' becomes 'att' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'involving' is an insertion indicator.
'fields' becomes 'leas' (I've seen this before).
'att' enclosing 'leas' is 'AT LEAST'.
Can you help me to learn more?
(Other definitions for at least that I've seen before include "Anyway" , "No less than" , "Eat salt (anag) - if nothing else" , "however" , "'If nothing else (2,5)'" .)