Union leader in running blue Fiat — lovely! (9)
I believe the answer is:
beautiful
'lovely' is the definition.
(similar in meaning)
'union leader in running blue fiat' is the wordplay.
'leader' indicates taking the first letters.
'in' indicates putting letters inside.
'running' indicates anagramming the letters.
The initial letter of 'union' is 'u'.
'blue'+'fiat'='bluefiat'
'bluefiat' is an anagram of 'beatiful'.
'u' going within 'beatiful' is 'BEAUTIFUL'.
(Other definitions for beautiful that I've seen before include "Pleasing to the eye" , "Lovely" , "Attractive, good-looking" , "Pulchritudinous, gorgeous" , "Aesthetically very pleasing" .)