Endlessly big mushrooms providing muscle (6)
I believe the answer is:
biceps
'muscle' is the definition.
(biceps is a kind of muscle)
'endlessly big mushrooms' is the wordplay.
'endlessly' means to remove the last letter.
'mushrooms' becomes 'ceps' (cep is a type of mushroom).
'big' with its last letter taken off is 'bi'.
'bi'+'ceps'='BICEPS'
'providing' acts as a link.
(Other definitions for biceps that I've seen before include "Arm workers" , "Arm muscles" , "Type of muscle" , "Muscle that flexes the forearm" , "I bend the elbow" .)