Feared radical left's leadership of a union (7)
I believe the answer is:
federal
'union' is the definition.
(I know that union can be written as Federal)
'feared radical left's leadership' is the wordplay.
'radical' is an anagram indicator.
'leadership' suggests taking the first letters.
The initial letter of 'lefts' is 'l'.
'feared' is an anagram of 'federa'.
'federa'+'l'='FEDERAL'
'of a' is the link.
(Other definitions for federal that I've seen before include "like the US political system" , "Lad-free (anag)" , "Red leaf (anag.)" , "(Of a union or government) involving states voluntarily joined together for common purposes" , "National, in the US" .)