Union leader with politician - one about to become referee (6)
I believe the answer is:
umpire
'referee' is the definition.
(I know that umpire is a type of referee)
'union leader with politician one about' is the wordplay.
'leader' suggests taking the first letters.
'with' is a charade indicator (letters next to each other).
'politician' becomes 'MP' (Member of Parliament).
'one' becomes 'i' (Roman numeral).
'about' becomes 're' (regarding).
The first letter of 'union' is 'u'.
'u'+'mp'+'i'+'re'='UMPIRE'
'to become' acts as a link.
(Other definitions for umpire that I've seen before include "Kind of referee as in cricket" , "Sports official" , "One enforcing the rules in games" , "Adjudicator in games such as cricket, tennis etc" , "Person enforcing rules" .)