Fear led to reform of the union (7)
I believe the answer is:
federal
'the union' is the definition.
(I've seen this before)
'fear led to reform' is the wordplay.
'to reform' indicates anagramming the letters.
'fear'+'led'='fearled'
'fearled' is an anagram of 'FEDERAL'.
'of' is the link.
(Other definitions for federal that I've seen before include "Red leaf (anag.)" , "describing Stateside governance" , "Lad-free (anag)" , "Relating to central government in the US" , "In alliance" .)