Customary gathering of leaders of United States unions at Lexington (5)
I believe the answer is:
usual
'customary' is the definition.
(similar in meaning)
'leaders of united states unions at lexington' is the wordplay.
'leaders of' indicates taking the first letters.
The first letters of 'united states unions at lexington' is 'USUAL'.
'gathering of' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for usual that I've seen before include "Normal, customary" , "regarding customs?" , "Stock" , "Ordinary" , "Accustomed" .)