Say purchase mushrooms for muscle (6)
I believe the answer is:
biceps
'for muscle' is the definition.
(biceps is a kind of muscle)
'say purchase mushrooms' is the wordplay.
'say' indicates a 'sounds like' (homophone) clue.
'purchase' becomes 'buy' (buy is a kind of purchase).
'mushrooms' becomes 'ceps' (cep is a type of mushroom).
'buy' sounds like 'bi'.
'bi'+'ceps'='BICEPS'
(Other definitions for biceps that I've seen before include "Arm workers" , "Arm muscles" , "One operating arm" , "Muscle that flexes the forearm" , "Flexor muscle" .)