UK citizen stands by Labour leader boxed in by Trotskyite extremes, likely to snap (7)
I believe the answer is:
Here is my best explanation:
'likely to snap' is the definition.
'uk citizen stands by labour leader boxed in by trotskyite extremes' is the wordplay.
'uk citizen' becomes 'brit'.
'stands by' means one lot of letters go next to another.
'leader' suggests taking the first letters.
'boxed in by' means one lot of letters goes inside another ('box' can mean contain, as in to put in a box).
'extremes' means to remove the middle letters.
'trotskyite' with its middle taken out is 'te'.
The first letter of 'labour' is 'l'.
'l' put inside 'te' is 'tle'.
(Other definitions for brittle that I've seen before include "Easily cracked or snapped", "Hard but liable to break easily", "Liable to snap", "Type of toffee made with nuts", "Easy to break", "Hard confectionery made with nuts", "apt to snap", "Easily broken", "Liable to break easily" and "Hard but easily broken".)