Terribly angry about Labour leader? Get knotted! (6)
I believe the answer is:
gnarly
'knotted' is the definition.
('gnarly' can be a synonym of 'knotted')
'terribly angry about labour leader?' is the wordplay.
'terribly' indicates an anagram.
'about' indicates putting letters inside.
'leader?' says to take the initial letters (I've seen 'leader' mean this).
The initial letter of 'labour' is 'l'.
'angry' anagrammed gives 'gnary'.
'gnary' placed around 'l' is 'GNARLY'.
'get' acts as a link.
(Other definitions for gnarly that I've seen before include "Bad-tempered" , "presenting a challenge?" , "Knotted" , "(Of a tree) covered with knots" , "Knotty (wood)" .)