Garbo gets it! What nonsense! (7)
I believe the answer is:
rubbish
'nonsense' is the definition.
(I know that nonsense can be written as rubbish)
'garbo gets it' is the wordplay.
I cannot quite understand how this works, but
'gets' could be 'is' (I've seen this in other clues) and 'is' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'what' acts as a link.
Can you help me to learn more?
(Other definitions for rubbish that I've seen before include "Nonsense and garbage (7)" , "Waste material - nonsense" , "Awful" , "Nonsense; very bad" , "Dross - twaddle" .)