United holding lead in Championship previously (4)
I believe the answer is:
once
'previously' is the definition.
(I've seen this before)
'united holding lead in championship' is the wordplay.
'united' becomes 'one' (as in a phrase like 'we are one').
'holding' is an insertion indicator.
'lead in' suggests taking the first letters.
The initial letter of 'championship' is 'c'.
'one' placed around 'c' is 'ONCE'.
(Other definitions for once that I've seen before include "Just as a single occurrence" , "On a single (or past) occasion" , "Formerly; as soon as" , "Formerly - not repeated" , "Sometime" .)