A birdie shows one's good at golf? Poppycock! (6)
I believe the answer is:
parrot
'a birdie' is the definition.
(I've seen this before)
'one's good at golf? poppycock' is the wordplay.
I cannot quite understand how this works, but
'poppycock' could be 'rot' (I've seen this in another clue) and 'rot' is found within the answer.
'one' could be 'a' (a thing is one thing) and 'a' is found in the answer.
This explanation may well be incorrect...
'shows' acts as a link.
Can you help me to learn more?
(Other definitions for parrot that I've seen before include "Tropical bird to repeat mindlessly" , "Mimic like exotic bird" , "Copycat who does not understand words or acts imitated" , "Repeat without understanding" , "Recite" .)