### Clue: UK citizen stands by Labour leader boxed in by Trotskyite extremes, likely to snap (7)
### Answer: BRITTLE 
 ### Explanation: Definition: likely to snap
 BRIT(a British/UK citizen) **plus**(stands by) [**1st letter of**(… leader) "Labour" **contained in**(boxed in by) **1st and last letters of**(… extremes) "Trotskyite"].