Workers admitting union leader's relatives (5)
I believe the answer is:
aunts
'relatives' is the definition.
(aunt is a kind of relative)
'workers admitting union leader's' is the wordplay.
'workers' becomes 'ants' (worker ants).
'admitting' means one lot of letters goes inside another (admit can mean to welcome in or accept).
'leader' says to take the initial letters.
The initial letter of 'union' is 'u'.
'ants' going around 'u' is 'AUNTS'.
(Other definitions for aunts that I've seen before include "Elderly female relations" , "Parents' sisters" , "Relatives get an odd US tan" , "Relatives get bad US tan" , "Relatives get a peculiar US tan" .)