Nonsense ... it's worth fifty! (4)
I believe the answer is:
bull
I believe this is a double definition.
'nonsense' is the first definition.
(I've seen this before)
'it's worth fifty' is the second definition.
(the bullseye in darts is worth 50 points)
(Other definitions for bull that I've seen before include "Tripe" , "Bovine animal" , "rubbish" , "Large quadruped" , "One charging" .)