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 subsidiary indication.
'uk citizen' becomes 'brit'.
'stands by' is a charade indicator (letters next to each other).
'leader' indicates taking the first letters.
'boxed in by' is an insertion indicator ('box' can mean contain, as in to put in a box).
'extremes' says to hollow out the word (remove centre letters).
'trotskyite' with its centre taken out is 'te'.
The initial letter of 'labour' is 'l'.
'l' placed 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".)