At end of his tether, as Dan famously was? (9)
I believe the answer is:
desperate
I believe this clue is a double definition.
'at end of his tether' is the first definition.
(at the end of one's tether means desperate)
'as dan famously was?' is the second definition.
(as in the comic book character Desperate Dan)
(Other definitions for desperate that I've seen before include "Hopeless; extremely bad" , "Without hope" , "Having lost all hope" , "Adept seer (anag)" , "in a stew" .)