Endlessly bad-tempered old comedian (7)
I believe the answer is:
groucho
'comedian' is the definition.
(I know that Groucho is a comedian)
'endlessly bad-tempered old' is the wordplay.
'endlessly bad-tempered' becomes 'grouch' (I can't explain this - if you can you should give a lot more credence to this answer).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'grouch'+'o'='GROUCHO'
Can you help me to learn more?
(Other definitions for groucho that I've seen before include "One of Marx Brothers" , "A Marx; a club" , "One of Harpo's brothers" , "One of four comedian brothers" , "The Marx Brother with the moustache" .)