Union leader introduced to workers' relatives (5)
I believe the answer is:
aunts
'relatives' is the definition.
(aunt is a kind of relative)
'union leader introduced to workers'' is the wordplay.
'leader' says to take the initial letters.
'introduced to' is an insertion indicator.
'workers'' becomes 'ants' (worker ants).
The initial letter of 'union' is 'u'.
'u' placed inside 'ants' is 'AUNTS'.
(Other definitions for aunts that I've seen before include "Relatives get a peculiar US tan" , "Sally and others" , "Relatives get an odd US tan" , "Female relatives, usually older" , "Parents' sisters" .)