Endlessly huge mushrooms providing muscle (6)
I believe the answer is:
biceps
'muscle' is the definition.
(I know that biceps is a type of skeletal muscle)
'endlessly huge mushrooms' is the wordplay.
'endlessly' means to remove the last letter.
'huge' becomes 'big' (similar in meaning).
'mushrooms' becomes 'ceps' (I've seen this before).
'big' with its last letter taken off is 'bi'.
'bi'+'ceps'='BICEPS'
'providing' is the link.
(Other definitions for biceps that I've seen before include "Flexor muscle" , "I bend the elbow" , "Forearm-flexing muscle" , "Contractor" , "Arm muscles" .)