Rustic leaders of British union get tummy ache (7)
I believe the answer is:
bucolic
'rustic' is the definition.
(bucolic is a kind of rustic)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for bucolic that I've seen before include "Relating to rural life" , "Characteristic of country life" , "Pastoral poem" , "Of rural or pastoral life" , "Rustic - pastoral" .)