Giant mushrooms no good for muscle (6)
I believe the answer is:
biceps
'no good for muscle' is the definition.
'biceps' can be an answer for 'muscle' (biceps is a kind of muscle). I am unsure of the 'no good for' bit.
'giant mushrooms' is the wordplay.
'giant' becomes 'bi' (I can't explain this - if you can you should believe this answer much more).
'mushrooms' becomes 'ceps' (cep is a type of mushroom).
'bi'+'ceps'='BICEPS'
Can you help me to learn more?
(Other definitions for biceps that I've seen before include "Upper-arm muscle" , "One operating arm" , "I bend the elbow" , "Arm workers" , "Weightlifters use them" .)