Extremists will desert EU, old chap (7)
I believe the answer is:
russell
'chap' is the definition.
'extremists will desert eu old' is the wordplay.
I cannot quite understand how this works, but
'will' could be 'll' and 'll' is located in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for russell that I've seen before include "actress (Rosalind or Jane)" , "He did philosophy" , "Philosopher - prime minister - film director" , "He controversially filmed" , "Ken -, Eng. film director (Women in Love)" .)